(Reuters) - A locked track switch was blamed for the collision of an Amtrak passenger train with a freight train that killed two people and injured more than 100 in South Carolina on Sunday, raising questions about the delayed rollout of a system to prevent such crashes.

Emergency responders are at the scene after an Amtrak passenger train collided with a freight train and derailed in Cayce, South Carolina, U.S., February 4, 2018. REUTERS/Randall Hill

The head of the National Transportation Safety Board said a switch on the tracks, which the freight hauler CSX Corp CSX.O owns and operates, was padlocked in a position that steered the Amtrak train onto a siding near South Carolina's state capital Columbia, where it crashed into a stationary unoccupied CSX train.

Robert Sumwalt, the chairman of the NTSB, told reporters Monday at a press briefing that track signals were inoperative because they were being upgraded for positive train control or PTC, a system designed to prevent derailments or crashes caused by excessive speed.

Investigators have conducted interviews and retrieved preliminary data from the Amtrak locomotive, which showed the horn was sounded seven seconds before the recording ended, Sumwalt said.

Investigators want to understand why the switch was left in a position that sent the train to an occupied track, triggering Amtrak’s fourth fatal crash since early December

“Just because we might find that somebody made a mistake, doesn’t mean that’s the end of the investigation,” Sumwalt said.

In November, the NTSB criticized what it termed Amtrak’s “weak safety culture” after finding that a series of unsafe conditions led to an Amtrak train striking a backhoe working on railroad tracks in Chester, Pennsylvania, in 2016, killing two maintenance workers and injuring 41. Sumwalt declined to draw any conclusions about whether Amtrak’s recent incidents pose a “systemic issue.”

Amtrak President and Chief Executive Richard Anderson told reporters on Sunday that CSX was responsible for the wreck of Train 91 because of the locked switch. A CSX spokeswoman declined to comment Monday.

Two patients remained in critical condition and another two were in “serious” condition at Palmetto Health hospitals, a spokeswoman said Monday.

Two of the recent Amtrak crashes involved vehicles driving around gates and being struck by trains, according to Anderson and North Carolina police. One of those crashes occurred last week when a chartered train carrying U.S. Republican lawmakers struck a garbage truck in Virginia.

Anderson said the South Carolina accident would have been prevented by PTC. Investigators say many deadly U.S. train crashes could have been averted if the system was in place.

Last month, U.S. Transportation Secretary Elaine Chao urged U.S. railroads and transit agencies to take all possible measures to meet deadlines to install PTC.

“It is important to understand the factors that contributed to this tragic accident and how all stakeholders can ensure a safe and reliable rail system going forward,” the department said in a statement Sunday.

A deadly Amtrak crash in December near Seattle that killed three occurred on a section of track that did not have PTC operating and could have prevented the derailment, the NTSB said. The NTSB also said the Amtrak engineer on that train told investigators he misread a signal shortly before the incident.

In 2008, Congress mandated the implementation of PTC nationwide by the end of 2015, then extended that deadline until the end of 2018.