(AP) — Authorities are still investigating claims made by a man who said he killed a total of 16 people in multiple states, though a law enforcement official confirmed Sunday that he is the primary suspect in the killing of his ex-wife and three others found in a car in New Mexico.

Local law enforcement officials in New Mexico and in New Jersey are working with federal agents on the veracity of claims Sean Lannon provided during his confession, including that he killed 11 other people, the official said. The official, who had direct knowledge of the probe, could not discuss the investigation publicly and spoke to The Associated Press on condition of anonymity.

Authorities said in court documents that Lannon made the admission in a phone call to a relative, who told investigators he expressed remorse.

Investigators are searching missing persons records and other police reports to try to identify any potential victims and determine if he was telling the truth during the confession, according to the official. The investigators are still reviewing electronic records, communications, surveillance video and financial documents as part of the investigation and are working to try to track down additional witnesses in the case, the official said.

Law enforcement officials are also still trying to compile additional information to better understand why he was in St. Louis, where he was arrested on March 11, the official said.

The bodies of Lannon’s ex-wife and three other people were found on March 5 in a vehicle in a parking garage at Albuquerque International Sunport, New Mexico’s largest airport. It’s not clear how they were killed.

Lannon has already been charged in the death of Michael Dabkowski, whose body was discovered on March 8 in his New Jersey home, just south of Philadelphia, after a welfare check. Sean Lannon is accused of breaking in and beating the 66-year-old to death with a hammer, according to an affidavit.

Lannon says he was sexually abused by Dabkowski as a child, and Lannon’s lawyer said his client was provoked.