The National Transportation Safety Board (NTSB) has released a preliminary report with a few new details on the recent fatal school bus crash in Chattanooga, Tennessee, including confirmation that the driver had been in two earlier crashes since the start of the school year.

After having wrapped up its on-scene work in Chattanooga in early December, NTSB is continuing to analyze the incident at its headquarters in Washington. At this point, the agency has not identified a probable cause of the crash.

“All aspects of the crash remain under investigation as the NTSB seeks to determine the probable cause, with the intent of issuing safety recommendations to prevent similar crashes,” the agency said in the preliminary report.

Much of the information in the preliminary report has already been released by investigators. Six students were killed and 31 were injured in the Nov. 21 crash in Chattanooga. The school bus driver, Johnthony Walker, 24, received minor injuries. The bus, a 2008 Thomas Built 84-passenger Type D model, was operated by Durham School Services.

The crash occurred on a two-lane road with a 30 mph speed limit and an advisory speed of 25 mph for curves. According to NTSB’s preliminary report, Walker lost control of the bus and departed the roadway to the left as he passed the second of two curves. The bus then hit a utility pole, overturned, and crashed into a tree, which caused the roof of the bus to collapse inward.

NTSB does not give any indication of the speed of the bus in its preliminary report, but police in Tennessee have estimated that it was traveling at about 50 mph.

Walker had been employed with Durham since April 2016. NTSB said that at the time of the crash, he was licensed and had a current commercial driver medical certificate. His preliminary toxicology test results were negative for alcohol and illicit drugs.

NTSB said that since the beginning of the school year in August, Walker had had one reportable crash and one non-reportable crash — both while operating school buses. NTSB’s preliminary report also reiterates a previously stated finding that Walker had apparently deviated from his normal route when the November crash occurred. Why the school bus driver may have been off route remains unclear.

Meanwhile, NTSB investigators are continuing to collect and analyze evidence from multiple systems on the bus that were capable of recording and transmitting event-related data.

The agency’s preliminary report is available here.