Seven police officers were assassinated in about an hour's time in what authorities said Tuesday was a coordinated effort that followed months of relative calm in a border city stricken by drug-fueled violence. Two other officers and a convenience-store employee were injured.

Several people were held briefly for questioning after Monday night's attacks and released, said Jose Manuel Yepiz, spokesman for the Baja California state attorney general's office.

"There is very strong indignation in the police force," Julian Leyzaola, Tijuana's public safety secretary, told a news conference. "We do not know the motive. We do not know where these attacks came from."

After four officers were killed by gunfire outside the convenience store, police scanners hummed with "narcocorridos," or drug ballads. One voice threatened over the airwaves that 30 officers would be killed.

The officers — three men and a woman — were found amid more than 200 bullet shells, the attorney general's office said. Witnesses told authorities that the assailants' faces were covered and two got out of the vehicle to finish off their victims.

Jovanni Fabiani, 14, heard a hail of gunfire while playing football at a friends' house.

"There was a mountain of smoke," he said.

The boy said he arrived minutes later to find one officer riddled with bullets from head to toe, including a shot in the forehead. Another's body was partly was under a car with a hand over her mouth, as if she was trying to hide.

One officer was shot dead in each of three attacks that followed, including one at a police station that also left an officer injured, the attorney general's office said. Another officer was injured in an attack on a police booth.

One officer was killed on his police motorcycle, authorities said. Witnesses reported that he was attacked by assailants in at least two vehicles. Six bullet shells were recovered near his bike.

The killings come as Mayor Jorge Ramos intensifies an effort to rid the police department of corrupt officers.

The city has fired 248 police officers accused of corruption since Ramos took office in December 2006 and about 130 others are suspended pending review for possible dismissal, Leyzaola said in an interview last week. The city has 2,160 officers.

Leyzaola said last week that about 15 Tijuana police officers had been killed on the job during Ramos' administration.

Nine police officers were killed last year in neighboring Playas de Rosarito, a city of 130,000 people, including about 14,000 U.S. citizens, Mayor Hugo Torres said. Seven or eight were involved in drug trafficking, he said.

Monday's killings were one of the most brazen since a period of bloodshed that claimed more than 400 lives in the last three months of 2008. According to U.S. and Mexican authorities, Tijuana is a battleground for two drug traffickers — Fernando Sanchez Arellano, heir to the notorious but enfeebled Arellano Felix cartel, and Teodoro Garcia Simental, a renegade lieutenant who broke away in April 2008 in a shootout that killed 14 people.

Victor Clark Alfaro, director of the Binational Center for Human Rights, said Tijuana officers have been killed before but never so many in a single attack. He said the slayings may signal a new wave of violence in Tijuana, across the border from San Diego, California.

"The violence follows a pattern," he said. "It falls, then it rises, then it falls. Today we are witnessing the beginning of the curve rising again."

Nationwide, Mexico's drug violence has claimed more than 10,700 lives since 2006, when President Felipe Calderon launched an anti-drug campaign. About 45,000 soldiers have been deployed to drug-plagued areas.