California ? Mexican police announced the arrests Saturday of four men suspected of involvement in the killing of a U.S. Border Patrol agent as American investigators searched hospitals for gunmen who were possibly wounded in the first such shooting death in more than a decade.

Investigators said they have notified hospitals on both sides of the border to be alert for patients with suspicious or unexplained injuries. They said at least two people may have been wounded and left blood at the scene.

Agent Robert Rosas was killed while responding alone to a suspected border incursion near Campo, a town in rugged, arid terrain in southeastern San Diego County. He was shot in the head and body and was dead when other agents arrived, said Keith Slotter, special agent in charge of the FBI’s San Diego bureau.