A man arrested early Sunday on suspicion of shooting dead six people in the northern US state of Michigan was an Uber driver who may have been picking up fares just before the killing spree began.

Uber confirmed that the suspect, 45-year-old Jason Brian Dalton, was a driver for the ride-sharing company, adding he had passed a background check and had no criminal record.

"We have reached out to the police to help with their investigation in any way that we can," Uber's chief security officer, Joe Sullivan, said in a statement.

Dalton was apprehended at 12:40 am Sunday (0540 GMT). He is believed to be the gunman who opened fire at three different locations in the city of Kalamazoo, killing six people and seriously wounding two others, police said.

A woman was seriously wounded outside an apartment complex, two people were killed at a car dealership, and four more were shot to death at a chain restaurant where a teenage girl was seriously wounded.

Kalamazoo County prosecutor Jeff Getting said Dalton, who surrendered without incident during a traffic stop, will face murder charges in court on Monday.

There is "no reason to believe" more than one person was involved in the crime, Getting added.

Local resident Matt Mellen said he was Dalton's passenger just a few of hours before the rampage began, and was taken on a hair-raising ride.

"We were driving through medians, driving through the lawn, speeding along and when we came to a stop, I jumped out the car and ran away," Mellen told WWMT, a CBS affiliate.

"He wouldn't stop. He just kind of kept looking at me like -- 'Don't you want to get to your friend's house' and I'm like, I want to get there alive."

Mellen said he jumped out of the car at 4:30 pm, and called 911, the phone number for emergencies. He told his fiancee what had happened and she posted Dalton's picture on Facebook as a warning to others.

"We're looking into his connection to Uber and whether or not he was picking up fares in between the shootings," Kalamazoo Public Safety Chief Jeff Hadley said, according to CNN.

- 'Your worst nightmare' -

The shootings began at 6:00 pm.

"What it looks like is we have somebody driving around, finding people and shooting them dead in their tracks," Kalamazoo County Undersheriff Paul Matyas told local media.

"This is your worst nightmare, when you have somebody just driving around randomly killing people," he added.

The gunman's first victim was a woman who was with her three children outside an apartment complex when she was shot, he said.

She was seriously wounded but is expected to survive.

The gunman next struck at 10:00 pm, killing an 18-year-old boy and an adult male at a car dealership, according to Getting, described as father and son by Michigan Governor Rick Snyder.

The last and deadliest shooting came 10 to 15 minutes later at the Cracker Barrel restaurant where four more people were killed, according to Getting.

Video at the car dealership and the restaurant helped police identify the shooter's car, he said.

Dalton was arrested about two hours later when a sheriff's deputy saw his car leaving a local bar in downtown Kalamazoo.

"He followed the car. The Kalamazoo Department of Public Safety was able to mobilize people as a backup. They pulled the car over without incident. They were able to make an arrest of the suspect without any significant incident," Getting said.

Police seized a semi-automatic handgun and "additional evidence" from the car, the prosecutor added.

Dalton was "pretty even-tempered" during his arrest, Getting said.

Kalamazoo is located some 190 kilometers (160 miles) west of Detroit and has a population of 76,000.

- 'Hearts are broken' -

Snyder ordered flags lowered for six days starting Monday in honor of each victim and met with families of people who were hospitalized.

"Our hearts are broken for the victims' families and friends, and I join in mourning their loss," the governor said.

Democratic White House hopeful Hillary Clinton tweeted: "Our hearts are in Kalamazoo today."

"We must end this epidemic. Communities across the country are paying the cost for inaction" on gun control, she added.

The killings there were the latest in a string of mass shootings in the United States that include the December 2 massacre in San Bernardino, California that left 14 people dead, and the December 14, 2012 Sandy Hook school massacre in which 20 children and six adults were shot dead.

Gun violence claims the lives of about 30,000 Americans every year and mass shootings -- rare in most countries -- have been on the rise.

According to the tracking website gunviolencearchive.org, there were 330 mass shootings in 2015, up from 281 in 2014.