Denver police arrested a man on suspicion of first-degree murder after he allegedly used a 30-pound rock to kill another man in downtown Denver on Saturday.

Edgardo Jimenez-Alvarad, 30, faces a first-degree murder charge after he allegedly “smashed” the rock onto another man’s head in the 1300 block of Larimer Street just before 6:30 p.m. Saturday, the Denver Police Department said on social media.

A witness who placed the 911 call said the suspect, later identified as Jimenez-Alvarad, stomped on the victim, who was laying down, and “smashed” the rock onto his head before running away to the area of 14th Street and Lawrence Street, according to the probable cause affidavit for Jimenez-Alvarad’s arrest.

The victim was taken to Denver Health Medical Center, where he was pronounced dead just before 7 p.m. Saturday, according to the affidavit. The victim will be identified by the Denver Office of the Medical Examiner.

Investigators later identified the rock allegedly used in the assault, which was measured to be about a foot in length, seven inches in width and four inches in height, the affidavit says. The witness later said they estimated it to weigh about 30 pounds.

Jimenez-Alvarad was taken into custody without incident about 15 minutes after the original call, according to the affidavit. Police are investigating the incident as a homicide.