OMAHA, Neb. (AP) — Investigators say the blast that prompted evacuations near Union Pacific’s massive railyard in western Nebraska last week appears to be accidental, but it’s not yet clear what caused it.

The Nebraska State Fire Marshal’s office is working with the railroad and experts from the Federal Railroad Administration to determine what caused a stationary container carrying perchloric acid that can be used to make explosives as well as a variety of food and drug products to explode last Thursday in North Platte.

The chief investigator for the fire marshal’s office, Adam Matzner, said Tuesday that investigators haven’t found any sign of a criminal act connected with the explosion, so the incident appears accidental. But the investigation is ongoing.

A spokesman for the Federal Railroad Administration said Tuesday that he couldn’t provide any additional details about the agency’s investigation.

Union Pacific spokeswoman Kristen South said the railroad is cooperating with the state and federal agencies while conducting its own review of what happened and how rail employees responded to find ways to improve how it responds to future incidents.

The local fire chief has said he doesn’t expect any lingering issues or contamination concerns related to the explosion and fire because the acid inside the shipping container largely burned off and any residue was contained at the scene. The fire didn’t spread beyond the container that was sitting on top of the one that exploded.

An area of about a one-mile radius around where the explosion happened in the west end of Union Pacific’s Bailey Yard was evacuated for several hours Thursday afternoon as a precaution, but because the fire happened near the edge of the city of about 23,000 people only a small number of homes were affected. North Platte is located about 230 miles (370 kilometers) east of Denver and about 250 miles (400 kilometers) west of Omaha.

The explosion did generate toxic smoke that concerned authorities, but the impact was nothing like the massive fiery Norfolk Southern derailment that happened in eastern Ohio in February. That crash in East Palestine, Ohio, generated nationwide concerns about railroad safety and calls for reforms.

Union Pacific is one of the nation’s largest railroads with more than 32,000 miles (nearly 51,500 kilometers) of track crisscrossing 23 Western states.