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 was n'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 did n'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 do n'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 .
Unexplained , intermittent failures persist despite repairs , Metro official says . Passenger recalls loud bang , `` accordion-like '' collapse of floor , screams . Delegate chides NTSB for making high-cost , `` impossible '' recommendations . 9 people killed , scores injured in June 22 crash near Red Line 's Fort Totten station .