DETROIT — Under questioning from Congress, a doctor who used to lead the NFL's concussion committee stuck to his position that there is no proven connection between football head injuries and brain disease.

Members of the House Judiciary Committee expressed frustration at the oral and written testimony delivered at Monday's hearing by Dr. Ira Casson, a neurologist from New York and former co-chairman of the NFL's panel on head injuries.

"There is not enough valid, reliable or objective scientific evidence at present to determine whether or not repeat head impacts in professional football result in long-term brain damage," Casson said.

Lawmakers had made a big deal out of Casson's absence at an Oct. 28 hearing on the same topic, and they went after him in direct questioning Monday. Rep. Linda Sanchez, D-Calif., also took shots at the league. She has compared the NFL's stance to that of tobacco companies who denied a connection between smoking and lung disease.

"I find it really ridiculous that he's saying that concussions don't cause long-term cognitive problems," Sanchez said. "I think most people you ask on the street would figure that repeated blows to the head aren't good for you."

Sanchez noted that the league formed its concussion committee in 1994, and wondered aloud whether the league's recent moves on concussions took far too long to come about.

"It seems to me that the NFL has literally been dragging its feet on this issue until the past few years," Sanchez said, later asking: "Why did it take 15 years?"

Casson resigned as co-chairman of the NFL's committee on mild traumatic brain injury in November. In written testimony Monday, he laid out reasons why there were flaws in recent studies — including some funded by the NFL — examining football head injuries.

"Some have suggested that scientific evidence regarding the question at hand is conclusive and that there is no need for further research," Casson said in his prepared testimony. "I strongly disagree with that position."

He said more research must be done on the effects of performance-enhancing drugs on the brains of football players. Some lawmakers questioned other witnesses about possible steroid links.

NFL commissioner Roger Goodell was not present at Monday's hearing on the campus of Wayne State University.

In October, he was grilled by lawmakers about his league's concussion policies.

Since then, the league has instituted stricter return-to-play guidelines for players showing concussion symptoms; required each team to enlist an independent neurologist as an adviser; entered into a partnership with Boston University brain researchers who have been critical of the league's stance on concussions; and conducted tests on helmets. The validity of those tests was questioned by witnesses at the hearing.

Asked for his thoughts on those changes, Casson questioned the merits of the independent neurologist mandate.

"We don't know if these independent neurologists have expertise in head injuries," he said. "We don't know if their opinions are going to be independent and reliable and stand up to scrutiny."

Rep. Steve Cohen, D-Tenn., said he sensed that the league's recent moves could have resulted from concern about lawsuits, as in, "'What did we know and when did we know it?' — and that should be secondary to the health of the NFL players and the college players and the kiddie league players and the high school players," he said.

Addressing Joseph Maroon, a team doctor for the Pittsburgh Steelers and current member of the NFL concussion committee, Sanchez asked: "Why do you think it took that long for the NFL to bring about these rules changes — or am I just being crazy?"

Maroon replied: "I dispute your position that nothing has been done since 1994."

Sanchez also pressed Casson on whether the "concept of permanent brain damage and dementia following repeated blows to the head is a very well-established and generally accepted principle in medicine."

Casson refused to give a direct answer, and Sanchez's tone grew more exasperated when she asked whether he would not "agree on something most laymen, probably most physicians, would agree with."

"We can disagree," he answered.

At one point, Casson said, "I'm not saying concussions are good for you."

"Well," Sanchez said, drawing laugher in the packed conference room, "that's the strongest statement I've gotten you to say."

Another witness, West Virginia University brain researcher Bennet Oamalu, testified it "has been established since the early 20th century" that repeated blows to the head cause damage.

Another witness, NFL Players Association executive director DeMaurice Smith, told Congress the league hasn't shared the injury data it collected from every player from 2006-2008.

"We have written the NFL a letter asking to clarify whether they have given us all the data that they have available or only a portion of it," Smith said after his testimony. "As of today, I have received an answer to that letter."

Browne insisted the union has the same injury information that the league collected.

"We received an Oct. 12 letter requesting injury data and to the best of my knowledge we gave the union the info they requested," Browne told the AP.

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AP Sports Writer Howard Fendrich in Washington contributed to this report.