Story highlights Bus driver was "going too fast," police officer testifies

The driver faces vehicular homicide, reckless endangerment and reckless driving charges

Chattanooga, Tennessee (CNN) 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 in killed six children.

School bus driver Johnthony Walker arrives at a court hearing Thursday.

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