A man has exposed himself to women on the Iron Horse Regional Trail in the Walnut Creek area six times since March, and police are warning trail users to be alert.
The suspect is described as a white male in his 20s, 5 feet 10 inches tall with a thin build and shoulder-length dirty blonde hair, according to the East Bay Regional Park District Police Department.
Witnesses have seen him wearing a baseball cap and, on some occasions, riding a BMX-type bicycle.
No violence has been reported, according to police.
Police are urging women to walk or run with a partner, remain aware of their surroundings and report any suspicious people to police.