Police salute as an ambulance arrives at the medical examiner's office carrying the body of Chicago Police Department Officer Samuel Jimenez, who was killed during a shooting at Mercy Hospital. Picture: Armando L. Sanchez/Chicago Tribune via AP

Washington - The killings were separated by thousands of miles but shared a common theme: brutal spasms of violence that shook U.S. cities at the beginning of the holiday season.

They included an execution-style quadruple slaying in the basement of a gentrifying Philadelphia neighborhood; an act of domestic violence that started as an argument in a Chicago hospital parking lot and ended with a doctor, a policeman and a pharmacy resident dead; and a mystery that flummoxed police, after someone fired randomly into a crowd of homeless people in Denver's downtown.

By the end of the day Monday, police in four major metropolitan areas across three time zones were investigating 11 homicides.

Philadelphia

Police on Monday found four people dead in the basement of a gentrifying neighborhood in the southwest part of the city. The victims - two men and two women - had been shot in the head in what authorities described as an execution-style slaying.

"All four of these individuals were executed; there are no ifs, ands or buts about it," Police Commissioner Richard Ross told ABC affiliate WPVI. "I can't get my mind around the fact that you have people walking around capable of such vicious acts."

One of the victims lived in the house, police said.

Investigators have not released the name of a suspect or determined how the killer or killers managed to get all four victims into the basement.

"The house is not in disarray. It doesn't look like there was a struggle upstairs. Just looks like they were either taken down there or they were already down there for some reason and they were shot. It doesn't appear to be a struggle," Ross said.

St. Louis County

Just outside St. Louis on Monday, a gunman burst into a family-owned Catholic Supply store, sexually assaulted several women, then fatally shot one woman in the head before fleeing, police said.

"We have a loose, armed gunman out there who's already shot one person," St. Louis County Police Sgt. Shawn McGuire told the St. Louis Post-Dispatch, saying the shooting was particularly brazen.

A 911 caller said a man entered the store, pointed a gun at the women and ordered them to take off their clothes before committing sexual assault and opening fire.

The suspect remained at large on Tuesday.

Chicago

A man showed up at Mercy Hospital on Monday, got into an argument with his former fiancee, and then killed her, police and witnesses said. In the ensuing chaos, he also shot and killed a Chicago police officer and a pharmacy resident.

The incident began with an argument just after 3 p.m. in the hospital's parking lot between 32-year-old Juan Lopez and his ex-fiancee, Tamara O'Neal, an emergency room doctor ending her shift. When a friend attempted to step in, the man lifted his shirt and displayed a gun in his waistband.

The gunfire began moments later, leaving O'Neal dead.

The man later exchanged gunfire with responding officers, shooting and killing Chicago Police Officer Samuel Jimenez and a pharmacy resident who walked out of an elevator at the wrong time, Dayna Less.

Lopez also died at the scene.

"The city of Chicago lost a doctor, pharmaceutical assistant and a police officer, all going about their day, all doing what they loved," Mayor Rahm Emanuel said, fighting back tears, according to the AP. "This just tears at the soul of our city. It is the face and a consequence of evil."

Denver

At least one gunman opened fire on a crowd of people two blocks from the stadium where the Colorado Rockies play, killing one person and sending four to the hospital, according to Denver Police.

Investigators haven't released a suspect's name or a motive, and were still trying to determine whether there was more than one shooter.

A witness told the Denver Post the shooter held a gun with both hands as he fired at a crowd of people.

Boston

Two people were killed and a third was injured after a shooting and stabbing at a Boston housing complex across the street from a children's centre, according to Fox affiliate WFXT.

Police have not said what was at the root of the deadly altercation that rattled the neighborhood and occurred as children were outside.

The shooting happened a week after hundreds of students walked out of Boston Public Schools and staged a "die in" outside the mayor's office, according to the Boston Globe. The students lay on the floor for 43 minutes, WFXT reported, one minute for each killing by a gun in the city in 2018.

The Washington Post