The operator of the school bus in last week’s fatal crash in Chattanooga, Tennessee, is cooperating with investigators to determine what happened, the company’s chief executive said.

In a video statement, Durham School Services CEO David Duke issued an emotional apology to the families impacted by the crash, in which six students were killed.

“I can’t fathom the anguish of the parents whose children were involved in this horrific accident, and it involved one of my company’s buses,” Duke said. “Nothing that I can say can take away the pain and the grief for these families. What I can say is that I’m deeply sorry for the children that were taken from their families with so much life before them.”

The leader of the contracting company said that his responsibility now is to look for answers about why the crash occurred and “how we can make sure that this never, ever happens again.”

Duke said that he directed his team to cooperate fully with the investigations by the National Transportation Safety Board (NTSB) and local authorities. He said in the statement that he couldn’t elaborate on the details of the crash or the investigations.

In a press briefing in Chattanooga on Wednesday evening, NTSB Chairman Christopher Hart confirmed that Durham has been “very cooperative” with investigators.

Hart said that according to the Motor Carrier Management Information System, Durham was operating on a conditional safety rating beginning in 2007, meaning that the Federal Motor Carrier Safety Administration had found some deficiencies, but those were resolved to the point that the company’s rating was upgraded to satisfactory in August 2015.

“We will be exploring what were the deficiencies and what was done about those deficiencies,” Hart said. “So we’re still in the process of reviewing their operational oversight and crash history.”

Durham is a division of National Express LLC, which is the second-largest school transportation contractor in the U.S. and Canada. The company operates about 21,500 school buses, transporting more than 1 million students for about 500 school districts.

In Chattanooga, Durham provides transportation service for the Hamilton County Department of Education. In a letter to Hamilton Interim Superintendent Dr. Kirk Kelly, Durham pledged to cover funeral costs, medical bills, and other expenses related to the crash. 

Watch David Duke’s video statement on the Chattanooga crash below.