Uber said Monday it was devastated by a deadly shooting spree by one of its drivers but had no plans to change its background check methods.

In a phone briefing with reporters, Uber expressed confidence in how well it probes whether aspiring drivers have pasts that signal trouble ahead.

Uber's chief security officer Joe Sullivan said the ride-sharing company was "devastated" and has been working "around the clock" with police since Saturday night's killing spree in Kalamazoo in the US state of Michigan.

"No background check process would have made a difference in this case because he did not have a criminal history," Sullivan said during a media conference call.

"If there is nothing on someone's record, a background check is not going to raise a flag."

Suspect Jason Brian Dalton, 45, sat stony-faced in an orange prison jumpsuit, thick glasses shielding his downcast eyes, as a judge on Monday read the charges against him in a Kalamazoo court. Dalton appeared via videoconference from the jail.

Dalton was formally charged with six counts of murder after he allegedly went on the weekend killing spree -- possibly picking up passengers along the way.

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

- 4.73-star rating -

Kalamazoo's public security chief 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.

Dalton cleared a background check to become an Uber driver on January 25, and was at the wheel for slightly more than 100 trips by users of the smartphone-based ride-sharing service during the following weeks, according to Uber.

Dalton had a rating of 4.73 based on a five-star system that passengers use to rate Uber drivers and "generally speaking" had received favorable reviews, Sullivan said.

Uber got some complaints on Saturday, and earlier, about Dalton's driving but nothing regarding violence or weapons, according to the San Francisco-based company.

Uber automatically suspends drivers after accusations of violence, but opts to discuss driving gripes because "we get a lot of complaints about bad driving and they are not all accurate," Sullivan said.

Uber did not see a need to modify its driver security screening process.

- 'Extremely safe' system -

While Uber does not have fingerprints of aspiring drivers checked against US criminal databases, it obtains extensive personal information including social security numbers to dig into records on local, county and national levels, according to the company.

"When it comes to understanding what criminal record someone has, we think we do a pretty good job," Sullivan said of Uber background checks, which involve going to courthouses and digging through files by hand if they are not available online.

Uber can also assess drivers with help of rider reviews and GPS tracking of each trip.

"As it stands right now, the system that Uber has is extremely safe," said Uber safety advisory board member Ed Davis, whose 35-year career in US law enforcement included serving as Boston police commissioner.

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

- No US 'panic button' -

Uber also said it did not plan to bring to the US a "panic button" being tested in the company's smartphone application in India to let riders quickly connect with police.

"People with smartphones here could just call 911," Sullivan said, referring to a special number that connects callers directly with police dispatchers.

"We can't hope to compete with that."

Uber shrugged off questions related to why the shooting spree made the firm a target for scrutiny instead of Dalton's prior employer or even the accused killer's ability to get a gun.

"I do think the fact that Uber is a technology company and a company that has been expanding so rapidly has made more of the media attention directed on Uber," said Uber safety advisory board member Margaret Richardson, a former advisor in the office of the US Attorney General.