MEXICO CITY (Reuters) - A gunman in a clown costume shot and killed the oldest brother of one of Mexico's most notorious drug trafficking families in the resort of Los Cabos, authorities said on Saturday.

Francisco Rafael Arellano Felix, 63, a former leader of the Tijuana Cartel, was shot in the head late on Friday at a family gathering in the southern tip of the state of Baja California Sur, a spokesman for state prosecutors said.

"A person dressed as a clown took his life," he said.

Local media reported that the killer had two accomplices, but this was not yet clear, the spokesman said. The gunman fled the scene.

Arellano Felix spent nearly 15 years behind bars for drug-related offenses after his arrest in Mexico in late 1993. Extradited to the United States in 2006, he was later paroled and walked free on his return to Mexico in early 2008.

Another official working with state prosecutors said Arellano Felix, the oldest of the brothers who headed the gang, was not wanted by authorities at the time of his death.

The possibility that his killer had ties to organized crime was being investigated, the official said.

Led by a large family of brothers and sisters, the Tijuana Cartel was one of the most powerful drug gangs in Mexico until it was gradually weakened by the capture or killing of several its leading members during the previous decade.

Arellano Felix's younger brothers, Francisco Javier, Benjamin and Eduardo are serving prison sentences in the United States. Another of his brothers, Ramon, was killed in a shootout with police in 2002.

(Reporting by Dave Graham; Editing by Doina Chiacu)