The driver in a Chattanooga school bus wreck that killed six children last week has declined to be interviewed by federal crash investigators on advice of his lawyer.

The National Transportation Safety Board said Thursday that it has filed subpoenas to obtain 24-year-old driver Johnthony Walker's medical and mental health history. Investigators are also seeking his cellphone records and employment details from his second job with Amazon.

The NTSB's final report is expected to be presented within 18 months.

The six children died on Nov. 21 after the bus carrying 37 students hit a mailbox and utility pole, rolled onto its side and then crashed into a tree.

Walker's attorney, Amanda Dunn, has said she expects him to plead not guilty if a grand jury indicts him for vehicular homicide.