The key suspect in the slaying of a 20-year-old pregnant Marine told comrades he would flee to Mexico to avoid a conviction on rape charges, and authorities said Wednesday the FBI is now working with Mexican authorities to track him down.

A wide-ranging manhunt for Cpl. Cesar Armando Laurean began last week, after authorities said he fled North Carolina and left behind a note in which he admitted burying the body of Lance Cpl. Maria Lauterbach, who had accused him of rape. Detectives later found Lauterbach's burned remains in a fire pit in his backyard.

Court documents filed this week by the FBI state Laurean told members of his Marine Corps unit he would flee to Mexico if it appeared he would be found guilty of rape. Laurean's wife also told authorities she believed he would head to Mexico if he was in trouble.

Click here to see photos from the case.

Click here to view FBI wanted poster for Cesar Armando Laurean.

"We strongly suspect, but have not confirmed, that Laurean may be in Mexico," said FBI spokesman Richard Kolko in Washington. "We have a strong working relationship with law enforcement partners in Mexico and we're working with them to locate and apprehend him."

Laurean, 21, of Las Vegas, is a naturalized U.S. citizen who was born in Mexico and who still has some family there, authorities said. The court documents are included with an FBI criminal complaint charging Laurean with unlawful flight to avoid prosecution. He is also wanted in North Carolina on a state arrest warrant for murder.

Laurean appears to have mailed letters back to his wife in North Carolina, according to two law enforcement officials who spoke on condition of anonymity because of the ongoing investigation. It was not immediately clear what the letters said or how many of them were sent. At least one was postmarked from Houston, one of the officials said.

Authorities have said his wife, Christina Laurean, is cooperating with authorities and provided them with the note her husband left before skipping town. In that note, he said Lauterbach committed suicide by cutting her own throat. Authorities have rejected that claim, and autopsy results released Tuesday found she died of "traumatic head injury due to blunt force trauma."

Authorities said the exam failed to answer all the questions detectives have about Lauterbach's death, including whether she gave birth before her death and of the identity of the father.

Lauterbach alleged in May that Laurean had raped her in March and April. A pregnancy test soon after was negative, according to Marine Corps officials, but a test in June was positive. Doctors estimated the date of conception was May 14.

Corps officials said Lauterbach met with prosecutors in November and said she no longer believed Laurean was the father. They said her regimental commander was intent on taking the case to an Article 32 hearing, the military equivalent of a grand jury proceeding.

Col. Gary Sokoloski, the judge advocate general officer for the II Marine Expeditionary Force, said Tuesday that Laurean denied having any sexual contact with her. He also did not violate a military protective order directing him to stay away from Lauterbach and continued to report for work on time in the weeks after her disappearance, he said.

"At no time did she indicate that she was threatened by Cpl. Laurean," Sokoloski said Tuesday. "When she was asked if she felt threatened by Cpl. Laurean, she said she did not feel threatened."

Authorities believe Lauterbach was killed around Dec. 15. Marine officials have they attempted to find her after she failed to report to work on Dec. 17, but had evidence — including a note left for her roommate in which she wrote she was tired of the Marine Corps lifestyle — that led them to believe she left on her own.