Authorities have quietly filed a felony conspiracy complaint and issued an arrest warrant under seal against Michael Robertson, the former boss and lover of convicted murderer Kristin Rossum. Robertson, who has been living free and working as a forensic toxicology consultant in Australia since the murder of Rossum’s husband in 2000, could be arrested and held on $100,000 bail if he ever returns to the United States.

In a case that continues to draw national attention, Rossum, a former toxicologist for the county Medical Examiner’s Office, was sentenced to life without the possibility of parole in February 2003 for stealing drugs from her lab, using them to poison Gregory de Villers, then staging a suicide scene in their University City apartment by sprinkling red rose petals over his body. The cause of death, ruled to be acute fentanyl intoxication, has been challenged by Rossum’s appellate attorneys as recently as late August.

Fentanyl, a controlled narcotic and fast-acting painkiller that is 100 times stronger than morphine, is used to treat cancer patients and is also administered during short procedures such as colonoscopies. An overdose of fentanyl, which has become increasingly popular as a recreational drug, can cause respiratory failure.

I covered the case from arrest to sentencing for the San Diego Union-Tribune and went on to write the book Poisoned Love on the case. I’m often asked by readers, “What ever happened to Robertson? Did he get charged?” My answer has always been “no.” But for nearly the past seven years, I — along with news reporters who published that same information — have been misinformed.

As it turns out, the single count of “conspiracy to commit an act injurious to public” filed against Robertson, who ran the Medical Examiner’s toxicology lab at the time of de Villers’s murder, has been kept on the QT since November 3, 2006. If found guilty, Robertson, now 43 and living with a new wife and family in Brisbane, could face three years in county jail or state prison.

Before this complaint was filed, prosecutors labeled Robertson as an “unindicted co-conspirator” in the murder case, and the investigation into Robertson’s potential involvement remains open today. Under the law, charges of murder and conspiracy to commit murder are treated much the same.

“I assume that’s why they did it. Just to keep a file open in case anything ever comes up in the future,” said Chuck Goldberg, Robertson’s local criminal defense attorney. “And I’m confident nothing ever will come up.”

Goldberg said he assumed the charges were filed “to preserve jurisdiction so that the statute of limitations wouldn’t run out.” But, he added, “We believe the statute had already run before they filed this.”

Goldberg sounded surprised when I called him about the charges in early September, because he said neither he nor Robertson was ever notified.

“I negotiated with the DA’s office, and they knew I represented him,” he said.

Steve Walker, communications director for the DA’s office, said it’s common when an arrest warrant is issued — even a murder warrant — not to notify the suspect, and when charges are filed, “We’re not legally required to notify the defendant or his attorney.”

Asked if the District Attorney’s Office had kept the conspiracy charge under wraps so Robertson could be caught unawares and arrested if he decided to come back to the United States, Walker declined to comment.

“Because this is a pending case, we are unable to discuss it,” he said.

He also said he couldn’t confirm whether prosecutors filed the conspiracy charge as a “placeholder” or a stepping stone toward hopefully filing more serious charges later, or whether any move to extradite Robertson had been undertaken.

Even the de Villers family didn’t know about the complaint.

“I knew it would be difficult for Michael Robertson to return to the U.S. but was unaware of the details regarding his arrest warrant,” Greg de Villers’s brother, Bertrand, wrote in an email.

According to the felony complaint, “On or about November 6, 2000, Michael David Robertson did unlawfully conspire together and with Kristin Rossum to pervert [and] obstruct justice and the due administration of the laws; to wit: by obstructing the investigation into the murder of Gregory de Villers.”

The complaint lists 77 “overt acts” of how Robertson and Rossum worked together “to cover up the true circumstances surrounding the murder.” The complaint was signed by detective Laurie Agnew of the San Diego Police Department, who has since retired. But it was not signed by any particular prosecutor. The warrant was issued by John M. Thompson, the trial judge in the Rossum case, who also approved the request to seal Robertson’s warrant affidavit.

Goldberg said the DA has taken no steps to extradite Robertson from Australia, where he returned to tend to his mother, who was ill with cancer, a month before Rossum was arrested in late June 2001. Robertson has since gotten divorced and remarried.

I emailed Robertson personally for his reaction, but he did not want to comment. Robertson has always maintained his innocence in de Villers’s murder, however, saying he doesn’t want to elaborate for fear it will be “misinterpreted or misrepresented.”

“I really would like to be able to say more, but it’s just wise at this stage to get on with things, try and put this behind me as best I can, and move on,” he told an Australian radio reporter in 2004. “And if my circumstance does change I may at some future stage be more than willing to comment on the turn of events that have unfolded over the last few years. But, at this stage, I’m just unwilling to take the risk of saying anything on the record that again may come back to haunt me, so to speak.”

The felony complaint was filed more than three years after Detective Agnew and Dave Hendren, one of the trial prosecutors, traveled to Melbourne, Australia, in the summer of 2003, by which time Hendren’s co-counsel, Dan Goldstein, had become a superior-court judge. Hendren is now the DA’s North County branch office chief.

Working with Detective Inspector Chris Enright in Melbourne, Hendren and Agnew tried to uncover new information to flesh out their suspicions that Robertson was involved in the murder, or at the very least in the cover-up. However, Robertson refused to meet with them on his attorney’s advice, and he put out the word to friends and colleagues to stay silent as well. Hendren and Agnew returned to the U.S., frustrated.

It is possible that the sealed warrant affidavit includes new details about the investigation, but Thompson agreed with the prosecution that “there exists an overriding interest that outweighs the right of public access to the record” and “a probability that the overriding interest will be prejudiced if the record is not sealed.”

The overt acts, listed in chronological order, describe dozens of examples of how Robertson interfered with and tried to influence the investigation into the murder; worked with Rossum to minimize their romantic relationship to investigators as simply “emotional,” and continued to strategize with her how to cover up the murder.

The complaint cites the exchange of 259 phone calls between Robertson and Rossum — as many as 11 in a day — from November 25, 2000, the day after Robertson was interviewed by police, through January 4, 2001, when police executed their search warrants.

After Robertson told investigators on January 10, 2001, that he was no longer in contact with Rossum, he exchanged 59 more phone calls with her. And soon after he returned to Australia in May, he sent Rossum a supportive note, saying, “I love you and take you with me in everything I do and wherever I go… My love is deeper, stronger and more wonderful than I have ever known before.”

Other acts authorities allege Robertson committed:

• Robertson lied to his superior, Lloyd Amborn, saying that Robertson and Rossum were in a coffee shop on the afternoon of November 6, 2000, the day de Villers was murdered, when in fact he met Rossum “very close to the crime scene.”

• The day after the murder, while Rossum was at her parents’ house in Claremont, east of Los Angeles, Robertson presented the case of de Villers’s death during the morning meeting at the Medical Examiner’s Office. He also logged on to Rossum’s password-protected computer that afternoon.

• Robertson interfered in the investigation by opening the container of autopsy specimens, which was stored for 36 hours in an unlocked refrigerator. He also falsely advised the medical examiner’s staff that based on his view of the specimens’ red coloring, “it appear[ed] that cough syrup played a role in the death of Gregory de Villers.”

• The final overt act alleged in the complaint occurred on November 4, 2002, when, “again in conformance with her agreement with Robertson,” Rossum, testifying under oath at trial, falsely denied stealing any drugs from the lab and also contended that de Villers died as a result of suicide after ingesting aspirin, cough medicine, and some of her old medications.

During Rossum’s trial, the prosecutors and defense attorneys both used Robertson as a key component of their cases, conveniently pointing to an empty chair in the courtroom where they said he would have been sitting if he hadn’t fled home to Australia.

Why weren’t these charges against Robertson ever announced? And why, when asked specifically about the Robertson investigation over the years, have officials at the District Attorney’s Office refused to comment or point me or the media to this public court file?

A few weeks ago, I talked with deputy district attorney Gary Schons, a new prosecutor on the case who has been responding to Rossum’s recent attempts to get a new trial. When I asked him to confirm a tip I’d gotten that Robertson had been charged, he told me he didn’t know anything about it, so I asked him to check “the file” for any documentation to make sure. He said he saw nothing in the file — the Rossum paper court file. New to the District Attorney’s Office since this complaint was filed, Schons was among those who learned of its existence after my call last week, saying his previous statement had been an “honest mistake.” Michael Robertson has his own file now, with a separate case number.

District attorney spokesman Steve Walker, asked in 2011 for any update on the case and the status of the investigation into Robertson’s involvement, would say only that, “We can’t comment on a pending investigation.” Last week, when I asked him why the district attorney never announced these charges or pointed anyone to the public file, he said that question fell under the same parameters.

In 2011, Agnew, with whom I’ve stayed in touch, said she tried to check with the District Attorney’s Office about granting me an interview for an updated edition of my book, which was released in December 2011. But after getting no response, she declined. When I emailed her this week about the charges, she said she still couldn’t comment without the district attorney’s permission.

San Diego police spokesman Gary Hassen also did not respond to a voice-mail message I left in May 2011, asking for an update on the case. I’ve been told by producers from several TV crime shows that they’ve gotten no cooperation from authorities on this case in recent years. And when authorities have been interviewed, it has only been on the condition that they not talk about the pending Robertson investigation.

The irony is that getting a copy of the complaint — with the exception of the sealed affidavit — was as simple as pulling a public court file. But that paperwork sat for years in the bowels of the downtown courthouse. Though technically a public document, its existence was known only to a small group from the prosecution team.

Over the past few years, Rossum’s appeals have made their way through the courts and her case has crept back into the news as recently as a couple of weeks ago.

Rossum’s new attorney, Elizabeth Missakian, is trying to find evidence to show that she can obtain meaningful results from testing de Villers’s autopsy specimens for fentanyl metabolites, with the goal of proving that they were contaminated — purposely or inadvertently — after he was already dead. Missakian is also hoping to get her client a new trial by proving that Rossum had inadequate trial counsel because her attorneys didn’t ask for this testing to be done at the time. Judge Thompson recently denied — without prejudice — Rossom’s motion to obtain documents related to the specimens as well as to access the specimens for metabolite testing.

Schons said that after 13 years, such testing wouldn’t prove that the specimens hadn’t degraded over time or that the metabolites were there in the first place. Nor, he said, would it prove that Rossum didn’t kill de Villers with one or both of the other two drugs found in his body, oxycodone or clonazepam.

“Pick your poison,” he said.

Meanwhile, the de Villers family waits and watches. “As for Kristin and her appeal, I trust state and/or federal attorneys will work hard to ensure her conviction and sentence are upheld,” Bertrand de Villers emailed.

Missakian said she had no comment on the Robertson conspiracy charge.