A lockdown at Acalanes High School was lifted around 12 p.m. Thursday after Lafayette police detained a suspect they had been searching for.
Police announced at about 10:30 a.m. that someone had fled into the school area and was being chased by Pleasant Hill police officers as well K-9 units and a helicopter.
It is unclear why the suspect, who was taken into custody near the school campus, was being sought.
Police said area residents are allowed to resume regular activities.
No additional information was immediately available.