Authorities say that a suspected contract killer charged with killing nine people in central California has confessed to committing up to 40 murders over several decades while working for drug cartels.

Jose Manuel Martinez, 51, reportedly told officials that he carried out the slayings working as an enforcer for Mexican drug cartels, said Errek Jett, the district attorney in Lawrence County, Ala. Jett said they believe Martinez because of the details he gave investigators about the murder scenes, reports the Associated Press.

Martinez was arrested last year after crossing the border from Mexico into Arizona before he was sent to Alabama where he is currently in custody and awaiting trial on one murder charge. While there, he confessed to killing one man, then to two more murders in Florida, and eventually to 40, including 11 in California. After the word got out, a number of investigators from across the country came to question Martinez, Jett said.

His defense attorney, Thomas Turner, described Martinez as being anxious to start his trial in June in Alabama so he can return to California. Turner said that his client maintains his innocence to the charge there and doesn't seem to be a cold blooded killer.

"I've found him to be polite and a likable individual," Turner said, according to the Washington Post. "He has a good personality as far as talking with him."

Martinez is also charged with lying in wait, kidnapping and murdering for financial gain, which makes him eligible for the death penalty.

It wasn't a drug deal that landed Martinez in an Alabama jail, but rather a personal vendetta that he developed against Jose Ruiz. He is accused of shooting Ruiz in the head for insulting his daughter and then dumping Ruiz's body in the woods in Lawrence County, Ala., back in March 2013.

Lawrence County Alabama Sheriff Gene Mitchell told ABC News, "In the course of interviewing him on [the Ruiz] murder, we developed a good rapport." Mitchell continued, "He felt comfortable talking to our investigator and in the course of those conversations, he mentioned he had done some other things."