OAKLAND -- A "specific and credible criminal threat" against Kaiser Oakland prompted an increased police presence Tuesday morning at two Broadway medical facilities, and advisories were sent to members and employees to show up later in the day, authorities said.

Police would not describe the nature of the threat, but Officer Frank Bonifacio said investigators were "actively working to identify and locate those responsible."

The threat was apparently received sometime Monday. That night, police started monitoring activity outside hospital buildings on Broadway. An increased police presence was in the area on Tuesday.

A statement issued Tuesday by Kaiser said the hospital was cooperating with police on the "potential security issue," and had delayed opening two medical office buildings at the Kaiser Permanente Oakland Medical Center until 12:30 p.m.

A statement from the company's communications office said that "our members, patients and employees are our highest priority and we are working with the Oakland Police Department to ensure their safety at all times."

Buildings with delayed openings were the Specialty Medical Office Building at 3600 Broadway and the Broadway Medical Office Building at 3701 Broadway.

The delay affected several medical offices, clinics, laboratories and pharmacies in the two structures. Parking garages adjacent to the offices were also closed until 12:30 p.m., officials said.

Kaiser's hospital and emergency department, along with other buildings at the Oakland Medical Center campus were open on Tuesday.

The two buildings will open at their normal times on Wednesday, according to the communications office.

Contact Natalie Neysa Alund at 510-293-2469. Follow her at Twitter.com/nataliealund. Follow Harry Harris at Twitter.com/HarryHarris15.