Washington - Toyota's top U.S. executive told lawmakers on Tuesday that he is not certain the company has fixed its runaway car problems even though it has recalled millions of vehicles around the world.

Although the automaker has blamed obstructing floor mats and sticky gas pedals for reports of cars accelerating out of control, lawmakers at a hearing on Capitol Hill appeared skeptical about the sometimes-conflicting accounts of what has gone wrong.

"Do you believe that the recall on the carpet changes and the recalls on the sticky pedals will solve the problem of sudden, unintended acceleration?" Rep. Henry Waxman, D-Calif., asked James E. Lentz III, president of Toyota Motor Sales USA.

"Not totally," Lentz replied.

"What do you need to do?" Waxman asked.

"We need to continue to be vigilant and continue to investigate all of the complaints that we get from consumers - that we have done a relatively poor job of doing in the past," Lentz said.

The hearing before the House Energy and Commerce Committee was jammed with spectators and media from the United States and Japan, who heard emotional testimony about terrifying car rides, as well as engineering analysis and a lengthy corporate apology. Lentz at one point fought back tears recalling that he lost his brother in a car crash years ago.

"There's not a day that goes by that I don't think of that, so I know what these families go through," he said.

In the end, the hearing outlined but did not resolve the controversy that has rocked the world's largest automaker in recent weeks: What exactly has caused Toyota cars, in rare but occasionally fatal instances, to rev and accelerate out of control?

At another hearing today, Toyota President Akio Toyoda plans to face the House Oversight and Government Reform Committee and attribute the company's troubles to rapid growth.

"I regret that this has resulted in the safety issues described in the recalls we face today, and I am deeply sorry for any accidents that Toyota drivers have experienced," he said in prepared remarks released ahead of the event.

On Tuesday, lawmakers first heard testimony from a retired Tennessee social worker whose Toyota 2007 Lexus ES 350 sped up to 100 miles per hour during a wild, six-mile ride in 2006. The car sped on even after the driver applied the emergency brake and shifted into neutral and then reverse.

"I prayed for God to help me," Rhonda Smith testified tearfully. "I called my husband on the Bluetooth phone system. I knew he could not help me, but I wanted to hear his voice one more time."

For months afterward, the couple tried telling their story to journalists, to Toyota and to federal safety regulators. They said they wanted to warn others of what they viewed as a deadly flaw. They also wanted their money back on a car that had only 2,728 miles on it.

One Knoxville television station, WATE-TV, ran a story.

Toyota looked at the engine but said it seemed fine.

An investigator from the National Highway Traffic Safety Administration (NHTSA) visited the couple in Tennessee, but he "seemed to arrive with the preconceived idea to sell to us that it was a floor mat problem," Smith testified. She was sure it was not.

It wasn't until the death of a California Highway Patrol officer and his family in August, which was preceded by their fraught 911 call seconds before the crash, that the company and federal regulators seemed to give more credence to consumer complaints.

"Shame on you, Toyota, for being so greedy," Smith said. "And shame on you, NHTSA, for not doing your job."

Lentz, who testified later, reacted to her story.

"Listening to Mrs. Smith, I'm embarrassed for what happened," he said. "You didn't have to have a death to understand the terror she had from that accident."

The Tennessee couple was followed by David Gilbert of Southern Illinois University, a professor who said his interest was "piqued" after seeing media reports about the runaway Toyotas. Based on 30 years teaching and testing automotive technology, Gilbert said he conducted an experiment that showed a Toyota Avalon's onboard computer could be tricked into operating as if nothing was wrong even if the accelerator was stuck and the car was speeding uncontrollably.

"To be quite honest, at the moment I discovered this, I was sick at my stomach," he said.

Gilbert's research was attacked by Rep. Steve Buyer, R-Ind., who has a Toyota assembly plant in his district. He asked Gilbert if he had "cut three wires" in the car to rig his test. Gilbert responded that he had not; he tapped into the wires with a monitor for electric current, a procedure within scientific methods.

Rep. Diana DeGette, D-Colo. told Gilbert that Toyota lawyers said they were able to duplicate Gilbert's experiment, but that it amounted to "sabotage." She asked Gilbert if the conditions simulated in his experiment could occur while driving.

Yes, Gilbert responded. Then why wasn't Toyota able to find this earlier?

"Maybe they weren't asking the right questions," Gilbert said.

Some lawmakers suggested that Gilbert's research was meant only to make money. Gilbert said he'd been paid $1,800 so far, by safety consultant Sean Kane, who works on behalf of victims' families and trial attorneys.

That Gilbert could so quickly turn up a problem that an entire industry has been looking for seemed "too good to be true," Lentz said.

"It's not in our interest, if a problem exists, to not find it," Lentz reminded lawmakers.

The other key question facing lawmakers is why the NHTSA did not act earlier in response to consumer complaints.

In late-afternoon testimony, Transportation Secretary Ray LaHood told lawmakers, "You have my 100 percent commitment we will get in the weeds on this."

Rep. Jane Harman, D-Calif., who owns between $116,002 and $315,000 in Toyota stock with her husband, recused herself from participating in Tuesday's hearing.

Harman International Industries, her husband's company, also has held lucrative contracts with Toyota for years, equipping cars with Harman Kardon audio systems.

Staff writers Kimberly Kindy and Dana Hedgpeth contributed to this report.