BALTIMORE >> The driver of a Baltimore school bus involved in a deadly crash with a commuter bus this week should not have been at the wheel because his commercial driver’s license was suspended two months earlier.

Glenn R. Chappell’s one-year Medical Examiner’s Certificate expired Aug. 31, and the Maryland Motor Vehicle Administration did not receive an updated one, as required by federal law for holders of commercial driver’s licenses, spokesman Chuck Brown said in a statement. These licenses are required for school bus drivers in Maryland.

Brown provided copies of two warning letters the agency sent Chappell. One, dated July 17, reminded him that his certificate would soon expire. The other, dated Sept. 8, told Chappell, “you are no longer authorized to operate a commercial motor vehicle.”

He was told to either submit updated information or have his permit downgraded to a noncommercial driver’s license.

Chappell also could have been kept from driving for another reason: He pleaded guilty in 2012 to second-degree assault. A State Board of Education regulation says a school system “may not” permit someone convicted of a violent crime to operate a school vehicle.

Neither Chappell’s employer, AA Affordable Transportation, nor Baltimore City Public Schools answered questions posed by The Associated Press in calls and emails.

A statement from AA Affordable, obtained by WJZ-TV, offered condolences to the victims’ families and said the company is “cooperating fully” with investigators and cannot comment during the investigation.

Brown said that to renew a certification, drivers must say whether they have experienced a brain injury, seizure, high blood pressure or dizziness, and submit to measurements of other standard health indicators. The resulting certificate doesn’t contain this information, however, so the agency doesn’t know about any medical conditions a driver might have.

Brown said the agency sends warning letters only to commercial driver’s license holders, not their employers. However, federal regulations require employers to maintain the certificates, which includes the date of expiration, for all of their drivers with commercial licenses, Brown said.

“Responsibility to maintain this certificate is on the individual and the employer,” Brown said.

Chappell, 67, was killed Tuesday, along with a Maryland Transit Administration bus driver and four mass transit passengers, when his school bus crossed the center line and smashed into the commuter bus after hitting a car and a roadside pillar in southwest Baltimore. No children were aboard.

Chappell’s son Moses said family members, like investigators, are awaiting his father’s autopsy results, which could show if he suffered a medical emergency. Investigators are also looking into whether speed played a role in the crash.

Moses Chappell said he never saw his father drink alcohol, he maintained a healthy lifestyle, and that “day to day,” he didn’t seem to have any health issues.

Moses Chappell said his father drove taxi cabs, trucks and buses over the years, and loved driving.

“It’s been the toughest 48 hours of my life,” he said.

A close reading of the board of education rules suggests that Glenn Chappell’s assault conviction wouldn’t have automatically disqualified him from driving a school bus; the wording appears to give authorities some discretion.

Board of Education spokesman Bill Reinhard said the local school district would have been responsible for making sure Chappell met state requirements, even though he worked for a contractor>