A 31-year-old man was shot and killed overnight Sunday, becoming the fourth homicide victim in Milwaukee during the Memorial Day weekend.

The outbreak of violence left another 14 people wounded in separate shootings, a stark contrast to last year's holiday weekend when only one homicide occurred.

So far, 40 people have been killed in homicides in Milwaukee, compared to 42 at the same time last year. The grim tally is far less than this time in 2015 when 61 people had been killed.

The Milwaukee Police Department "is doing everything it can to apprehend those who have been involved in these shootings," Mayor Tom Barrett told reporters Monday afternoon.

Police have arrested suspects in three of the homicides and are searching for a known suspect in the fourth, Barrett said. The victims' names have not been released.

Of the nonfatal shootings, it appears four were related to robberies and seven stemmed from an argument or fight, he said. Police aren't sure what led to the other three shootings.

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Two of the shootings wounded children. An 8-year-old boy was wounded in a double shooting Sunday afternoon after a fight broke out in the street, according to police.

On Saturday, a 7-year-old girl and a 27-year-old woman were wounded in the 3400 block of N. 67th St. The woman was arguing with someone when a gun was fired, police said.

Most shootings were not random incidents

Although the investigations are ongoing, Barrett said most of the shootings appear to involve people who know one other or have some connection.

Many of the people involved had a criminal record, and several of the shootings involved people buying or selling drugs or felons with guns, Barrett said.

"The majority are not random acts of violence," he said.

So far this year, 208 people have been wounded in shootings and have survived — the same number of victims at this time last year.

Barrett said Milwaukee police will continue to have "many people on the streets" working to arrest the shooters.

"We, of course, are depending on others in the criminal justice system to make sure that the most violent ones are behind bars," he said.

Barrett said people appear all too willing to carry guns into situations they know could end in conflict. Police have already seized nearly 1,000 guns this year, he added.

"If you're a mother or father or grandparent and you've got a young person in your life and they're telling you they need to take a gun to protect themselves, my response is maybe you shouldn't be going there," he said.

"Maybe you should stay away from those situations."

Erin Richards of the Journal Sentinel staff contributed to this report.

Correction: An earlier verison of this post incorrectly stated there were no homicides during Memorial Day weekend last year.