Two police cars apparently lured into an intersection in the violent city of Ciudad Juarez on Friday were ambushed by gunmen. At least seven officers were killed in the brazen midday attack, along with a 17-year-old passerby.

Two other injured officers were hospitalized and under heavy guard, said public security spokesman Enrique Torres, to prevent gunmen from attempting to finish the job.

All but one of the dead officers were from the U.S.-trained federal police force. The seventh was a municipal policewoman. A contingent of 5,000 federal police agents took over security of Juarez two weeks ago as part of a plan to phase out the army, which has occupied the city — Mexico's deadliest — for two years. The death toll has skyrocketed over that period.

Bullet-pocked police trucks remained on the street for some time after the ambush, surrounded by bodies covered in white sheets.

As drug-war violence continued to intensify, sucking in innocents caught in the wrong place at the wrong time, dozens of people were killed across Mexico on Thursday and Friday.

All told, according to government statistics, more than 22,700 people have been killed since President Felipe Calderon launched a military-led offensive against well-armed and powerful drug cartels shortly after he assumed office in December 2006.

Juarez has long been one of the focal points of ruthless warfare among rival gangs. Hundreds of people have been killed this year alone in the besieged city just across the Rio Grande from El Paso. A U.S. Consulate official, her husband and the husband of another consular employee were among the victims last month.

Public Security Ministry officials said in a statement that Friday's ambush on federal officers may have been in retaliation for a round of arrests this week, including eight suspected of trafficking who were seized Thursday.

The two patrol units attacked were apparently beckoned to a stop by a vendor, the officials said, and then gunmen poured from the rear of waiting vehicles and opened fire. They shot from at least two sides, then escaped. One of the assailants may have been wounded.

By late afternoon, no suspects were reported arrested.

The identity of the 17-year-old civilian killed in the shooting was not immediately released.