Around 4:20 p.m., San Francisco sheriff's officials and San Bruno police alerted school personnel about a report of somebody with a rifle heading toward the campus, located at 3300 College Drive, spokeswoman Barbara Christensen said.
The community college is adjacent to a San Francisco County Jail facility.
The report was that the armed person had been seen in the Sweeney Ridge park behind the campus, she said.
As of 5:15 p.m. the campus remained on lockdown.
A man answering the phone at the Skyline College Bookstore at about 5 p.m. said the bookstore was in lockdown.
"We just took in some people, right now we're just making sure that everyone is safe," he said before abruptly hanging up.
A lieutenant at the San Mateo County Office of Emergency Services said he received an alert advising people on campus to stay away from windows and lock all doors.