Two people slipped into a U.S. zoo and beat a monkey to death, according to reports.
Police are hoping that a baseball cap found in the U.S. zoo will help them track down the suspects in the beating death of the monkey.
A robbery at the zoo in Boise, Idaho, left a Patas monkey dead of blunt force trauma to the head and neck.
Police are analyzing blood found at the scene to determine if it was the monkey or one of the two human intruders.
Police said that two men wearing dark clothes were seen by a security guard outside the fence near the display of the primates exhibition.
Both suspects fled and police have been unable to find them.
But they said late on Saturday that a gray baseball cap with a skull design was probably left behind by one of the suspects. Police have not said why the suspects beat the monkey to death.