Two bicyclists are dead and a third was seriously injured after authorities say a sheriff's deputy plowed into the riders while on patrol in Cupertino.

Santa Clara County Sheriff's Sgt. Don Morrissey said the deputy accidentally crossed over the center line at around 10:25 a.m. Sunday and struck the three cyclists, who were riding in the other way.

Morrissey said the deputy immediately stopped, began performing first aid and called for paramedics. One of the cyclists was pronounced dead at the scene.

The other two riders were taken to Stanford Medical Center with major injuries. California Highway Patrol Officer Tracy Hoover said a second cyclist was confirmed dead about an hour later.

The deputy has been placed on leave while the CHP investigates the crash.