An alleged contract killer charged in California with nine murders has confessed to murdering 40 people in a career as a drug cartel enforcer.

Jose Manuel Martinez, 51, allegedly told investigators he had worked for a drug cartel since he was 16, collecting debts and pocketing 25% of the money.

Prosecutors said they believe Martinez, because of the details he gave during his confession.

The alleged hitman targeted victims in California between 1980 and 2011, said Tulare County assistant district attorney Anthony Fultz.

Fultz added he is confident Martinez committed at least the nine killings he is charged with.

Fultz declined to comment on any connection Martinez may have had with drug cartels, saying he did not want to damage the case at this early stage. He added that the additional murders Martinez confessed to will be investigated further.

Investigators have released some details of the murder cases, saying six of the victims were killed in Tulare County including one in Santa Barbara. They were aged between 22 and 56.

One man was shot dead in 1980 driving to work in the morning, while two men were shot in 1982 working on a ranch, one survived.

The same year, ranchers found a man shot and stabbed to death.

Another victim was found in 2000 shot to death in bed with his four children at home.

Martinez was arrested after he crossed the border from Mexico into Arizona in 2013.

In addition to the murder charges, Martinez was charged with lying in wait, kidnapping and murder for financial gain.

If convicted, he would be eligible for a death sentence.

Sheriff Mike Boudreaux said investigators are looking at other unsolved homicides for possible links to Martinez.