The head of Volkswagen's U.S. business endured tough questions from a congressional panel on Thursday as he apologized for the emissions cheating scandal that has nearly crippled the German car giant.

"On behalf of our company, and my colleagues in Germany, I would like to offer a sincere apology," Michael Horn told a House Energy and Commerce subcommittee. "These events are deeply troubling. I did not think that something like this was possible at the Volkswagen Group."

Volkswagen has admitted that 11 million of its cars worldwide, including about 500,000 in the United States, were outfitted with a "defeat device" that allowed it to cheat on emissions tests. It had stopped selling its diesel vehicles in the United States, and, Horn said Thursday, would not sell certain model-year 2016 diesel cars in the United States anytime soon.

That was not enough to satisfy lawmakers. "Ultimately, the saying rings true: Cheaters never prosper," said Rep. Fred Upton (R-Mich.), chairman of the House Energy and Commerce Committee. "That's why we are here today."

Lawmakers pressured Horn to describe when he found out about the defeat device, repeatedly citing a 2014 report by West Virginia University researchers that initially indicated there was a problem.

Horn, who at times appeared frustrated by the onslaught of questions, said he was not made aware of the problem until about Sept. 3. The West Virginia University report merely indicated an emissions problem, he said, not evidence of cheating.

Rep. Joe Barton (R-Tex.) appeared unpersuaded.

"Do you really believe," Barton said, "[that] as well run as Volkswagen has always been reputed to be, that senior-level corporate managers and administrators had no knowledge for years and years?"

Although Horn conceded that it was "hard to believe," he insisted he had no knowledge of the circumvention software. Horn said he felt personally deceived by the incident, which he blamed on a couple of individuals at the company.

Rep. Chris Collins, a Republican from New York, rejected the idea that the use of a defeat device was not a corporate decision.

"Either your entire organization is incompetent when it comes to trying to come up with intellectual property, and I don't believe that for a second, or they are complicit at the highest levels in a massive cover-up that continues today," Collins said.

EPA officials are expected to weigh potential penalties for VW as part of its investigation into the company's efforts to circumvent emissions tests. Volkswagen last month admitted to putting special software in its diesel cars that instructed the vehicles to emit less pollution during testing. On the open roads, however, the cars created up to 40 times the nitrogen oxide allowable by law. Untreated diesel exhaust has been identified as a significant contributor to respiratory illness, particularly among asthma patients, the elderly and children.

The crisis has forced at least one high-profile resignation, with Volkswagen AG chief executive Martin Winterkorn stepping down days after the scandal broke. Since then, Volkswagen shares have plummeted 40 percent and resale values on VW diesels have dropped on average some 13 percent, or about $1,700 per car.

The scandal has dealt a severe blow to VW's credibility, and nearly 3 in 4 consumers think the issue could spread beyond Volkswagen to other automakers, according to a Kelley Blue Book poll. More than half of the respondents said they have either lost "complete" faith in VW or now generally mistrust the company.