MANUELITO, N.M. (AP) — The Federal Railroad Administration is now leading the investigation of last month’s fiery train derailment in New Mexico.

National Transportation Safety Board officials said Thursday that the FRA will determine the probable cause of the April 26 derailment.

NTSB investigators have examined the track and derailed tank cars and will focus on the performance of the tank cars and emergency response actions.

The derailment of an BNSF Railway freight train forced a 50-mile stretch of Interstate 40 to close and led to two days of precautionary evacuations for 52 Manuelito residents who lived within a two-mile radius of the derailment site.

No injuries were reported, McKinley County officials said.

The train had left Belen, New Mexico, and was headed to Phoenix and was traveling about 53 mph at the time of the derailment, well below the maximum authorized speed of 70 mph near the derailment site, NTSB investigators said.

Of the 35 cars that derailed, six tank cars were carrying non-odorous propane, the NTSB said.

Four of the six tank cars were breached and released their contents, which ignited, investigators said.

A fifth tank car carrying propane — a type of liquefied petroleum gas — was exposed to the flames and released vapor through its pressure relief device, investigators said.

Each derailed tank car was carrying about 30,000 gallons of liquefied petroleum gas, according to investigators.