An Uber driver was charged with six counts of murder Monday after he allegedly went on a weekend killing spree in the US state of Michigan, amid reports he admitted to taking people's lives.

Jason Brian Dalton, 45, sat stony-faced in an orange prison jumpsuit, thick glasses shielding his downcast eyes, as a judge read the charges against him in a Kalamazoo court.

"I wish to remain silent," Dalton said via videoconference from the jail when asked if he had anything to tell the court.

At another court hearing, officials said Dalton told investigators that "he took people's lives" after he was read his rights, ABC News and CNN reported.

Prosecutors said they were still trying to determine why Dalton began firing -- seemingly at random -- as he drove through Kalamazoo Saturday night.

"That is probably the million dollar question: 'Why would this individual do this?'" Kalamazoo's public security chief, Jeffrey Hadley, told CNN.

The first victim was a young woman who was with her three children outside an apartment complex when she was shot at 6:00 pm. She was seriously wounded but is expected to survive.

Four hours later, the gunman opened fire at a car dealership, killing a father and his 18-year-old son.

The last and deadliest shooting came 10 to 15 minutes later at a restaurant, where four more people were killed and a 14-year-old girl was critically wounded, officials said.

Dalton was also charged with two counts of assault with intent to murder and eight weapons offenses.

Hadley described Dalton, a former insurance adjuster, as "an average Joe" who had no criminal record and had not come to the attention of law enforcement before the murders.

The New York Times said neighbors described him as quiet and polite, but that he occasionally fired a gun outside the back door of the house he shared with his wife and two children, ages 10 and 15.

His family released a statement offering their "deepest sympathies and condolences to the families of the victims" and vowed to "cooperate in every way that we can to help determine why and how this occurred."

"There are no words which can express our shock and disbelief," read the statement.

"While it seems woefully inadequate, we are deeply sorry and are praying for everyone affected."

- Uber assists police -

Police are looking into reports Dalton picked up fares between shootings.

A man who declined to be identified told CNN that he got a short ride from Dalton in the midst of the rampage.

"I said, 'You're not the shooter, are you?' He said no. I said, are you sure? He kind of just said, 'No, I'm just tired, I've been driving for seven hours.'"

Uber's tracking system is expected to help police trace Dalton's vehicle during the series of shootings.

The ride-sharing company said Dalton had passed a background check and had no criminal record prior to joining the company.

Uber officials told reporters Monday that they were devastated by what had happened but had no plans to change their background check methods.

"As it stands right now, the system that Uber has is extremely safe," said Uber safety advisory board member Ed Davis.

But "a background check is just that; it does not foresee the future. After an incident like this, we all struggle for answers," he added.

- Obama decries violence -

The Kalamazoo killings were the latest in a string of US mass shootings that include the December 2 massacre in San Bernardino, California that left 14 people dead, and the December 14, 2012, Sandy Hook school massacre that killed 26, including 20 children.

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

In what has become a grim tradition, President Barack Obama on Monday decried the nation's epidemic of gun-related deaths and said more needs to be done to keep guns away from "dangerous people."

"Their local officials and first responders did an outstanding job in apprehending the individual very quickly, but you've got families who are shattered today," Obama told a White House gathering of US governors.

"Clearly we're going to need to do more if we're going to keep innocent Americans safe."

Michigan Governor Rick Snyder ordered flags lowered for six days in honor of each of those killed and met with relatives of hospitalized victims.

"What a tough situation," he told reporters as he described watching the wounded girl's mother react as her critically injured daughter squeezed her hand. "Such senseless violence."

Dalton was ordered held without bail ahead of a preliminary hearing set for March 3.