Afterward Mendell briefly bowed his head, then backed away from claims he'd made in his prepared testimony, delivered under oath, that no ill cows from his plant had entered the food supply.
So-called "downer" cattle have been largely barred from the food supply since a mad cow disease scare in 2003 because they pose a higher risk for that disease and other illnesses, partly because they often wallow in feces.
The panel's chairman, Rep. Bart Stupak, D-Mich., asked Mendell whether it was logical to conclude from the videos that at least two downer cows had entered the nation's food supply.
"That would be logical, yes, sir," Mendell said.
"Has your company ever illegally slaughtered, processed or sold a downer cow?" Stupak asked.
"I didn't think we had, sir," Mendell said.
Asked about the discrepancy with his written testimony, Mendell said, "I had not seen what I saw here today." He said that the Agriculture Department had not shared with him some of the undercover video shot by the Humane Society of the United States.
Stupak pointed out that the video has been available on the Humane Society Web site.
After Mendell's testimony, his lawyer sought to clarify Mendell's remarks. Asa Hutchinson, a former GOP congressman from Arkansas who once led the Drug Enforcement Administration, said Mendell would not dispute logical conclusions drawn by Stupak about downed cattle illegally entering the food supply.
"But it can't be conclusive because he does not know all the facts of it, he hasn't studied it and he only saw one brief shot at it during his testimony," Hutchinson said.
Mendell was appearing under subpoena before the House Energy and Commerce investigative subcommittee. He was a no-show at a committee hearing last month.
It was Mendell's first public appearance since the undercover video led to his plant's shutdown and last month's beef recall, the largest in U.S. history. The recall stretched back two years, and Agriculture Department officials have said most of the meat has been consumed. Some 50 million pounds of the beef went to federal nutrition programs, mostly school lunches.
No illnesses have been reported, and Agriculture Department officials have insisted there is minimal risk. But Stupak noted that the incubation period for mad cow disease can be a dozen years or more.
Richard Raymond, Agriculture Department undersecretary for food safety, acknowledged "there is that remote possibility" that cases of mad cow could emerge years from now as a result of the Westland/Hallmark practices.
Raymond also said that the Agriculture Department had found evidence of more than the two non-ambulatory cattle shown in videos Wednesday improperly entering the food supply. Even though carcasses also undergo inspection and can be discarded after slaughter, "it's a reasonable statement to assume it did enter commerce, some of it," Raymond said.
Two workers from the Humane Society video were fired and are facing animal cruelty charges from San Bernardino County prosecutors in an ongoing criminal investigation. One of those workers has said he was just following orders while his supervisor has reportedly told police he was under pressure to ensure slaughter of 500 cattle per day.
Mendell said everyone at the plant was under pressure to do their job but that couldn't excuse abuses. He also disputed reports cited by lawmakers that the Humane Society's undercover investigator, who shot the videos with a hidden camera, didn't receive proper training in slaughter practices when he was hired at the plant.
Mendell gave the committee a form document signed by the investigator when he was hired acknowledging he'd received the requisite training. The Humane Society has declined to disclose the identity of its investigator, but on the training form he signed his name as Sean Thomas.
Mendell contended that there is good training at his plant and that he has a strong safety record and never previously knew of abuses like the ones on the Humane Society videos. "Obviously my system broke down," he said.
He said he's received death threats and has heard from people "praying for us to suffer and die like the cows."
"Our company is ruined. We cannot continue," Mendell said. Some 220 employees have lost or are about to lose their jobs, he said.