SAN DIEGO (Border Report) — Baja California’s State Attorney’s Office says its investigators are fairly certain Brayan Rivera is responsible for a fourth murder that occurred in Tijuana while he was in the city last year.

Rivera, a U.S. citizen, was arrested back in July outside his mother’s house in Downey, California, where FBI agents and U.S. marshals were waiting for him.

He is now the subject of extradition proceedings in federal court in Los Angeles.

The Mexican government wants Rivera returned to Tijuana to face murder charges in connection with the deaths of three other women who have been described as “sex workers.”

“We have identified one more case and have gone over the evidence; it points to him being responsible for the fourth murder,” said Ricardo Iván Carpio Sánchez, Baja’s Attorney General. “We have really delved into this deeply.”

Carpio Sánchez added they have evidence showing Rivera was in Tijuana when the murder of the fourth woman occurred, and that she was killed in the same way as the other three alleged victims.

The attorney general also said they remain optimistic Rivera will be brought back to Tijuana to face charges.

“We are in the initial phase now making its way through federal court in the United States, so far we’ve seen positive signs — he is going to exhaust all possible avenues in his defense before being sent to Tijuana,” he said.