WALNUT CREEK -- An unidentified man was killed when he jumped from the Treat Boulevard overpass onto southbound Interstate 680 shortly after 9:15 a.m. Tuesday, authorities said.

Sheriff's deputies were called to the scene when a witness reported a man standing on the overpass, apparently preparing to jump, according to Jimmy Lee, spokesman for the Contra Costa County Sheriff's Office. By the time they arrived, he had jumped onto the freeway, Lee said.

It was not immediately clear if he was struck by a vehicle, Lee said. A CHP dispatcher would not comment on the incident except to note that the three righthand lanes were temporarily closed during an investigation. The lanes were re-opened around 11 a.m.

The man was pronounced dead around 9:30 a.m., Lee said. There are no reports of other injuries.

Contact Daniel M. Jimenez at 510-262-2728. Follow him at Twitter.com/DMJreports.