WASHINGTON (CNN) -- The cause of a deadly Washington Metro subway crash last month remains a mystery, with one problem identified by investigators persisting even after engineers replaced a key part that was believed to be faulty, a top Metro official said Tuesday
Investigators are shown at the crash scene in Washington. Nine people were killed in the June 22 accident.
At a congressional hearing, Jim Graham said Metro replaced a device that was "fluttering," signaling the presence of a train one moment and not the next.
"You'd think that that would remedy the issue, that ... we would have solved the problem," said Graham, chairman of Metro's board of directors. "In fact, the new device ... continued the same fluttering as the former device."
"We're left with a very compelling mystery as to what is going on here," he said.
The National Transportation Safety Board, which is leading the investigation, acknowledged that Metro's system of detecting trains continues to have unexplained, intermittent failures.
"Sometimes it's working; sometimes it's not, even with those changes," NTSB member Deborah Hersman told a House committee.
"We've walked back the cable to see if there might be some cabling issues. There's a lot of challenges here and we're changing out some components and trying to identify what the problem is," she said.
Metro has said it will operate the subway trains on manual control until officials have identified and fixed the problem with the automatic train system.
Nine people were killed and scores were injured during the June 22 crash just north of the Red Line's Fort Totten station.
At Tuesday's hearing, a passenger on one of the trains gave a dramatic description of the accident.
Patrick Tuite of Kensington, Maryland, said he had just put down his newspaper and closed his eyes to relax when he heard a screeching sound, someone yelling and "one of the loudest bangs I've ever heard in my life."
Tuite said in the impact, he hit the seat in front of him.
When Tuite peered into the forward-most car, he saw extensive damage. The floor of the car had collapsed "accordion-like," he said.
"We could hear the people at this point in the first car," he said. "And it was pretty chaotic. They were screaming; they were upset," he said. But the collision had jammed the doors between the cars, making it impossible to reach victims in the front car.
It wasn't until he got off the train that he realized the first car was thrust into the air, he said.
Eleanor Holmes Norton, the District of Columbia's Congressional delegate, chided the NTSB for making prohibitively costly safety recommendations while ignoring cheaper, common sense solutions, such as putting the newer, more robust rail cars at the front and back of trains.
"Over and over again, you said, 'Do the impossible,' " Norton, a Democrat, said, referring to NTSB recommendations that Metro replace older cars with sturdier new ones. "Why didn't the transportation board at least recommend this rather low-tech, low-cost step?"
Hersman said the NTSB's role is to push for safety improvements.
"We do make recommendations, Ms. Norton, and we don't have to pay for them, so we recognize that frustration," she said.
"But our charge is not to [consider costs]. Our charge is to recommend what we think is in the best interest of the safety community. We are the conscience and the compass of the transportation industry, and they [transportation providers and regulators] get to decide if and how they implement it," Hersman said.