Bullet casings still littered the ground on Friday in Coatepec Harinas, a troubled municipality southwest of Mexico City, a day after 13 police officers on patrol were brutally murdered in an ambush by suspected gang members.

Forensic teams and dozens of heavily armed police and military officials gathered at the cordoned off crime scene with checkpoints in place after what was one of the worst mass slayings of Mexican law enforcement officials in years.

Some residents looked on from their homes, shaken by the violence that rocked the small town nestled between verdant hills and corn fields, where most people make their living cultivating crops.

Local police said crime is common there, but Thursday's ambush was particularly harrowing for local people, with bullets strafing some of their homes.

The convoy of security personnel was attacked in broad daylight by gunmen as it patrolled the area about 40 miles (64 km) south of the city of Toluca, authorities said.

The attackers rounded up the bodies of the fallen police officers into a pile and continued to spray them with bullets, according to a local officer at the scene on Friday.

"They finished them off," said the officer, who declined to give his name. He had lost colleagues in the ambush, he said.

Local resident Guadalupe Flores, 26, said she heard the shooting directly outside her window when it started.

She had just finished feeding her baby and, fearing for their lives, took the child and hid at the back of the house until the violence was over about a half hour later.

"I was terrified for my girl... It sounded horrible," Flores said, pointing at the bullet casings in her driveway and bullet holes on the cement wall of her home. Men cursed loudly at each other in her driveway during the shooting, she said.

Mexican President Andres Manuel Lopez Obrador on Friday vowed to bring the killers to justice.

"We are filled with sorrow about what happened in the State of Mexico," he said at a regular news conference, referring to the state where the killings occurred.

There would not be "impunity" for anyone, he added. (Reuters)

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