Federal investigators have completed their on-scene probe of the November bus crash in Chattanooga that killed six students and injured dozens after the bus slammed into a mailbox, a utility pole and then rolled onto its right side right before colliding into a tree, officials said.

National Transportation Safety Board officials, who wrapped up their examination last week, were able to recover the bus’s video records, the event-data recorder, or so-called “black box,” and the last 30 days of GBPS download for the bus and the routes driven by the driver prior to the crash.

The data was sent to Washington, D.C. for further review.

Investigators also confirmed that a recall was issued for the brake throttle valve, but the valve was installed as required.

Officials requested an interview with the driver, Johnthony Walker, but he declined. They did, however, issue subpoenas to local hospitals and mental health facilities to obtain his medical history.

They also sent a subpoena to AT&T for his cellphone records and one to Amazon for his employment record.

The NTSB, which conducts independent investigation of all major accidents, is expected to issue a preliminary report by the end of the month.

Chattanooga Police have said that Walker was driving "well above" the posted 30 m.p.h. speed limit on Talley Road when the bus overturned and crashed into a tree around 3:30 p.m. on Nov. 21.

Four girls and two boys from Woodmore Elementary School died.

Police charged Walker with multiple counts of vehicular homicide, reckless endangerment and reckless driving.