ATLANTA (AP) — The chairman of a House panel investigating the Gulf oil spill said Friday that BP won't let members talk to several employees who may have critical information about what led to the catastrophe.

Rep. Bart Stupak, D-Mich., told The Associated Press that BP PLC has cited its own investigation as its reason for denying access to the employees.

BP spokesman David Nicholas said in an e-mail that BP "has not objected to providing access to any of the specific BP employees that the committee has requested, and we continue to cooperate with the committee." He did not immediately respond to a request to explain what he meant by that.

BP was leasing and operating the Deepwater Horizon rig when it exploded April 20, killing 11 workers and blowing out the well that has now gushed as much as 131.5 million gallons of oil into the Gulf.

"They have been slow in bringing forth documents and witnesses we want to talk to," Stupak said of BP.

He also said information gathered so far shows it could be difficult for the government to prosecute anyone for the spill because of vague environmental laws and other challenges.

"And remember, in a criminal case you have to prove intent," he said. "That's very, very difficult in a situation like this."

Stupak said there are a half-dozen people his committee wants to question, but hasn't been able to. Stupak, chairman of the House Energy and Commerce Committee's Subcommittee on Oversight and Investigations, did not say whether some of those people work for companies other than BP that were involved with the Deepwater Horizon.

He also said he isn't ready to issue subpoenas yet.

Among the people Stupak's committee wants to talk to is BP well site leader Donald Vidrine, one of the top two BP officials on the Deepwater Horizon at the time of the blast.

Vidrine was scheduled to testify earlier this month at a hearing in Kenner, La., where government investigators were questioning rig workers. But Vidrine had a health problem and didn't testify, a Coast Guard official said at the time.

Vidrine told investigators three days after the explosion that at one point before the blast he had a call from the rig floor and that there was a problem "getting mud back" from the well. Some time later, there was an explosion, he said.

Stupak described Vidrine as an "important piece" of the puzzle as investigators try to determine what happened.

Vidrine has repeatedly declined to speak to the AP, and BP has ignored several requests for information on his status with the company.