(CNN) Thanksgiving week got off to a violent start Monday with four fatal shootings across the US in a matter of hours.

Attacks in a Chicago hospital, on a Denver street, at a suburban St. Louis business and in a Philadelphia basement left 10 people dead -- including one of the gunmen -- and at least four people wounded.

A gunman killed 3 at a Chicago hospital

A police officer, a doctor and a pharmacy resident were killed after a Monday afternoon shooting at Chicago's Mercy Hospital

The gunman, who had been in a relationship with the one of slain hospital employees, also died, police said.

It's not clear if he died from police gunfire or a self-inflicted gunshot wound.

The gunman approached one of the female victims -- a hospital employee -- around 3:30 p.m. in the hospital's parking lot, and an argument ensued. Police were called after the man revealed a handgun to a friend of the woman who had tried to intervene.

When officers responded, the gunman opened fire on them before they could exit their vehicle. Officers followed him into the hospital and exchanged gunfire with him, police said.

During the shootout, Officer Samuel Jimenez -- a father of three -- was fatally wounded. Also killed were emergency room physician Tamara O'Neal and pharmacy resident Dayna Less.

Chicago police officers and firefighters form an honor guard as the body of Police Officer Samuel Jimenez is taken to the coroner Monday, November 19, 2018, in Chicago.

5 people were shot, 1 fatally, in downtown Denver

One victim was pronounced dead at the scene and four people were taken to the hospital, all in a stable condition, the Denver Police Department said.

The shooting occurred just after 4 p.m. near Coors Field on Lawrence and 21st streets.

Police are talking to witnesses hoping to get a description of a suspect, authorities said.

Denver Police Department crime scene investigators look for evidence on Monday, November 19, 2018, near a shooting scene that left one person dead and sent four others to the hospital.

In Missouri, a sexual assault and fatal shooting

St. Louis County police are searching for a man believed to be armed and dangerous after a woman was shot at a Catholic Supply business in Ballwin, Missouri -- about 30 miles west of St. Louis -- Monday afternoon.

Officers called to the scene found her suffering from an apparent gunshot wound, the St. Louis County Police Department said in a statement.

The suspect committed a sexual assault and shot a woman at the business before fleeing the scene, police said.

The victim was taken to a hospital, and police later said on Twitter that the incident "is now a homicide investigation."

Authorities investigate the scene at a Catholic Supply store where police say a gunman shot a woman in the head on Monday, November 19, 2018, in Ballwin, Missouri.

4 people were found shot in the head in a Philadelphia basement

Police called to a home in Southwest Philadelphia midday Monday found the bodies of four people in the basement.

The victims -- two men and two women who appeared to have been in their 30s -- each were shot in the head, Philadelphia Police Commissioner Richard Ross told reporters.

Nadirrah Smith is escorted from the scene of a fatal shooting on Monday, November 19, 2018, in Philadelphia.

"All four of these individuals were executed," Ross said. "There are no ifs, ands or buts about it."

Preliminary information suggests the two men were related, he said.