A police officer walked into a doughnut shop Wednesday while it was being robbed and was shot in the head, authorities said, becoming the third officer in four days to be wounded by gunfire in this city.

The 54-year-old officer, Charles Cassidy, was checking on a Dunkin' Donuts shop around 10 a.m., Police Commissioner Sylvester Johnson said. The robber shot him almost immediately after the officer walked in.

"He actually walks into a robbery, a holdup, unbeknownst to him," Johnson said.

The bullet went through the officer's brain, the commissioner said. He was taken to a hospital, where doctors were "doing everything they can to save his life," Johnson said.

The robber left the shop, returned to pick up the officer's gun and then fled, Johnson said. Police swarmed the North Philadelphia neighborhood in a search for the gunman, combing the area on foot in squad cars and in a police helicopter. More than a dozen schools in the area were locked down.

Mayor John F. Street said he met with the officer's family at the hospital. "We told them we're going to catch the perpetrator of this act," Street said at a news conference.

The shooting came about 12 hours after a masked gunman shot Traffic Officer Mario Santiago in the shoulder during a chase downtown.

Santiago was responding to a report of a gunman in a sport utility vehicle shooting at another car, injuring two men and a woman, police said.

He was chasing the SUV when the gunman eventually got out of his vehicle and approached the squad car, firing twice through the window. Santiago was hit once in the right shoulder, police Commissioner Sylvester Johnson said.

Santiago survived and was in fair condition Wednesday.

The gunman in that shooting apparently jumped into the Schuylkill River, where searchers later recovered a body. Police said Wednesday morning that they had not determined if it was the suspect's body.

Early Sunday, an officer responding to a melee at a West Philadelphia nightclub was shot in the ankle. More than two dozen bullets were fired, police said. One suspect was fatally shot and another was arrested.