SAN FRANCISCO ” A tiger escaped from its pen at the San Francisco Zoo today, killing one man and injuring two others before police shot it dead, authorities said.

The three men were in their 20s; they were together and were not zoo employees, San Francisco Police spokesman Steve Mannina said.

They were attacked just after the 5 p.m. closing time outside the zoo’s Terrace Cafe on the east end of the 1,000-acre grounds.

It was unclear how the tiger escaped or how long it was on the loose. The zoo, which is open 365 days a year, was evacuated immediately after the attack was reported.

Police arrived to find the tiger on top of a victim. The tiger then started moving toward a group of approaching police officers, and they opened fire with handguns, Mannina said.

The two injured men were in critical but stable condition at San Francisco General Hospital, San Francisco Fire Department spokesman Lt. Ken Smith said. A call to the supervising nurse at San Francisco General was not immediately returned.

“This is a tragic event for San Francisco,” Smith said. “We pride ourselves in our zoo, and we pride ourselves in tourists coming and looking at our city.”

Authorities did not believe there were any other people attacked, but because it was dark they could not be certain. Investigators remained on the scene and Smith said a thorough sweep of the grounds would be conducted in the morning.

It was not immediately clear what kind of tiger was involved in the attack. The zoo has both Siberian and Sumatran tigers, according to its Web site.

Officials at first worried that four tigers had escaped, but only one got out of its pen, Mannina said.

Last December, one of the zoo’s tigers mauled a zookeeper during a regular public feeding. The 350-pound animal reached through the cage’s iron bars and badly lacerated her arm. The zoo’s Lion House was temporarily closed during an investigation.