The driver in a deadly school bus crash in Chattanooga, Tennessee, was speeding about 50 mph in a 30 mph zone, a police investigator said in court Thursday.

Officer Joe Warren of the Chattanooga Police Department’s traffic division testified that he knew he was “dealing with a bus that was going too fast.” Warren said he estimated the speed of the bus at between 50 and 52 mph, about 20 mph above the posted speed limit.

Warren also said an onboard camera video showed that bus driver Johnthony Walker was using his cell phone on the bus, but Warren did not say when that occurred. It is illegal for a school bus driver to use a cell phone once the first child has boarded the vehicle whether it is moving or not, Warren said.

At the close of Thursday’s hearing Hamilton County General Sessions Judge Lila Statom found there was enough evidence against Walker to send his case to a grand jury. The November 21 crash killed six children.

Walker faces vehicular homicide, reckless endangerment and reckless driving charges.

At the time, Walker’s attorney said the crash had been a “devastating tragedy” for the Chattanooga community. “There are families in mourning, there are families that are still dealing with loved ones in the hospital in intensive care,” attorney Amanda Dunn said.

“Mr. Walker’s family is also devastated by this tragedy. Mr. Walker has been devastated,” she said. “We’re hopeful that the investigation can conclude quickly and we have a better understanding of what occurred. …”

Here’s what we know about the crash so far.

How did the crash happen?

On the afternoon of Monday, November 21, the No. 366 school bus swerved off a street, toppled on its side and slammed into a tree.

The impact of the crash almost split the vehicle in half.

Investigators say the driver lost control of the bus while driving down an unapproved route at well over the speed limit.

According to the court affidavit, the charges were brought “because of the reckless nature of Mr. Walker’s driving, combined with his very high speed and weaving within his lane.”

Who was on the bus?

The bus was carrying 37 students from Woodmore Elementary School.

Six children — aged 6 to 10 — died after the crash. More than a dozen others were injured.

Some of those taken to the hospital were too young to know their last names and could only identify their mothers as “Mama.”

Who was Johnthony Walker??

Walker’s mother, Gwenevere Cook, told CNN her son was a respected man and the father of a 3-year-old son.

“He is a marvelous son. For two years he worked two jobs. He’s never been in trouble before,” Cook said.

Walker worked at Amazon as a fulfillment associate. The company said his last shift before the November 21 crash was an overnight on November 19.

What did parents think of the driver?

Going back to at least September, parents and students had complained to Hamilton County Department of Education employees about Walker’s alleged disregard for children’s safety, records show.

A principal expressed concern about Walker’s driving twice in the weeks before the crash, students claimed he was trying to injure them and a parent, responding to complaints he had cursed at children and tried to hurt them with his driving, threatened to beat Walker up, the records allege.

Meantime, Walker complained to officials that throughout the fall, saying students failed to heed his safety instructions and treated him with disrespect.

The accident was the second time in two months that Walker crashed a school bus. Police said no children were injured in the earlier “minor wreck.”

What does the bus company say?

In a YouTube statement, Durham School Services CEO David Duke said the incident left him filled with pain.

“I can’t fathom the anguish of the parents whose children were involved in this horrific accident and it involved one of my company’s buses,” he said. Duke said the company was cooperating with National Transportation Safety Board and local investigators.

In a later video statement, he said the company was bringing in additional safety measures including equipping buses with smart cameras that would activate and record when they sensed unusual driving, he said. It would also install a secure nationwide cloud-based complaint management system, he said.