PLASTER ROCK, N.B. -- A large fireball shot into the sky and a boom resounded Friday from the site of a train derailment in northwestern New Brunswick as officials carried out a controlled explosion to blast holes in three tanker cars carrying liquefied petroleum gas.

Thick plumes of black smoke billowed from the wreckage site after crews detonated explosive charges placed on the hulls of the cars in order to allow vapour and gas to burn off in a wooded area.

"Indications are that it proceeded as designed, but that is based on a helicopter inspection," CN spokesman Jim Feeny said after the explosion, carried out by the rail company and a crew from Louisiana with expertise in cleaning up derailed tankers.

The procedure, known as a vent and burn, was used on two tanker cars carrying liquefied petroleum gas that were burning prior to the explosion and a third one with the gas in it that was not on fire.

Feeny said ground crews have to wait for the site to cool off before checking out the site to determine whether all the liquefied petroleum gas was burned off.

He said the intent behind the explosion was to allow crews to move cars safely. It's the same type of explosion that was used following a CN train derailment in Gainford, Alta., in October.

Some tanker cars that derailed from the CN (TSX:CNR) freight train have been burning since Tuesday night in Wapske, near Plaster Rock.

Feeny said fires from diesel fuel in a locomotive that derailed and a tanker car that was loaded with crude oil were no longer burning.

An evacuation of the area has affected about 150 people since the derailment of 19 rail cars in the 122-car train, and there is no word on when they might be allowed to return to their homes. Some of them have been allowed to temporarily return to their residences.

"CN's focus is on dealing with the fire and then we'll give the information to the authorities who then decide what the situation with the evacuees will be," Feeny said.

Laurie Beaulieu, one of the evacuees, said she was not overly frustrated from being away from home and appreciates the work crews have to do.

"They have a job to do and I'd rather go home when it's safe than go back too soon and have to leave again," said Beaulieu, 41.

A senior investigator with the federal Transportation Safety Board said Thursday that they found a cracked wheel and broken rail at the scene of the derailment, but cautioned that it's too early to determine the exact cause.

Feeny has said that based on its preliminary investigation, the company believes a wheel and possibly an axle failure caused the derailment.

The province's Health Department said as a precaution, people in the area with private wells should not drink their water until after they have been tested.

Feeny said CN will cover the costs of the cleanup and compensate those whose properties have been damaged and incurred expenses as a result of the evacuation.