SAN JUAN, Puerto Rico (Reuters) - A Puerto Rico jury on Thursday rejected handing down the death penalty in the case of a drug dealer convicted of killing his girlfriend after he discovered she was an informant for the U.S. Drug Enforcement Administration.

Edison Burgos Montes was sentenced to life in prison for the 2005 killing of Madeline Semidey Morales, who cooperated with the DEA in its investigation of him.

The case was closely watched in Puerto Rico, where capital punishment is prohibited by the U.S. commonwealth's constitution. Had Burgos Montes been sentenced to death, he would have been brought to the U.S. mainland for execution.

The U.S. government has sought capital punishment cases in 17 states that also do not carry out executions, but many Puerto Ricans see its attempt to bring capital punishment cases against island criminals as an unwelcome federal imposition on local affairs.

The verdict came as the island prepares for a November 6 referendum which will ask Puerto Ricans whether they want to a remain a U.S. commonwealth or change the island's political status. Replying to a second question, voters can state their preference for statehood, independence or free association.

As the jury started deliberations earlier this week, Governor Luis Fortuno and Resident Commissioner Pedro Pierluisi, Puerto Rico's sole representative to the U.S. Congress, spoke out against the death penalty. Both members of the pro-statehood New Progressive Party are up for re-election on November 6.

Senator Alejandro Garcia Padilla, the pro-commonwealth Popular Democratic Party candidate for governor, said he would work to end federal capital punishment prosecutions involving Puerto Rico residents.

It was the fourth death penalty conviction sought by U.S. authorities since capital punishment was eliminated on the island in 1929, two years after the last execution.

The jury convicted Burgos Montes, 42, after a two-month trial of one count of conspiracy to possess with intent to distribute cocaine and a count of murdering a witness in retaliation for providing information to law enforcement, along with several other counts, according to the U.S. Attorney's office.

Under an agreement with the Fortuno administration, federal authorities have assumed jurisdiction in carjackings, public shootings and other lesser crimes, worried in part by the island's role as a transshipment point for the smuggling of illicit drugs from South America to the United States.

In 2011, the number of homicides in Puerto Rico rose to a record of more than 1,100. Federal prosecutions on the island have nearly doubled over the past year.

(Reporting by Reuters in San Juan; editing by Kevin Gray and Mohammad Zargham)