(Reuters) - The driver of a Baltimore school bus involved in a commuter bus crash that killed him and five others lacked authorization to drive because his medical certification had lapsed, a Maryland state spokesman said on Thursday.

Certification for the school bus driver, Glenn Chapell, 67, had expired at the end of August, Maryland Transportation Department spokesman Chuck Brown said in a statement.

Chappell failed to respond to two Department of Transportation letters about medical certification, Brown said. Federal law requires employers to have certification for licensed commercial drivers.

"Since your Medical Examiner's Certificate expired, you are no longer authorized to operate a commercial motor vehicle," a Sept. 8 letter to Chappell said.

Chappell was driving a school bus with no students aboard that police said smashed into an oncoming Maryland Transit Administration bus on Tuesday. Five people aboard that vehicle, including its driver, died at the scene, along with Chappell.

A spokesman for Chappell's employer, AA Affordable Transportation in Baltimore, declined to comment about his lack of certification.

There were no skid marks at the accident scene, and investigators are focusing on speed. An autopsy could shed light on whether Chappell had a medical problem at the wheel, but results may not be forthcoming for weeks, police have said.

Investigators have found recording equipment from the commuter bus but do not know if it is in usable condition, police said.

(Reporting by Ian Simpson in Washington; Editing by Dan Grebler)