PLEASANT HILL -- Diablo Valley College in Pleasant Hill will remain closed off with yellow police tape as officers stand guard for a second day in response to two bomb threats called in to the Concord and Pleasant Hill police departments Sunday night and early Monday morning, a Contra Costa Community College District police dispatcher confirmed.

A bomb squad spent the day searching the campus after bomb-sniffing dogs from the Walnut Creek Police Department found two suspicious locations during a 5 a.m. sweep of the school grounds: an outdoor locker on the campus quad and a second undisclosed location.

The bomb squad blew open the suspicious locker in a controlled blast Tuesday morning, but found it empty, according to college spokeswoman Chrisanne Knox.

The first bomb threat was called into the Concord Police Department at about 10 p.m. Sunday. The second threat was called into the Pleasant Hill Police Department at about 6:30 a.m. Monday. Knox described both threats as "non-specific," and said that neither one mentioned a specific time or a specific on-campus location. However, both mentioned DVC, she said.

On recommendations from the Department of Justice, the school had increased police presence on campus Monday, Knox said. Bomb-sniffing dogs were brought in Monday between 9 a.m. and 3 p.m., but did not find anything, Contra Costa Community College District police Lt. Thomas Sharp said.

Police from Concord, Pleasant Hill, Bay Area Rapid Transit, the Contra Costa Community College District; the FBI; and county sheriff's deputies are continuing to investigate the situation.

Dean of Student Life Bill Oye said the college had no choice but to take the threat seriously, but that this kind of incident is not unusual during exam periods.