Three United States Postal Service trailers in New Jersey went up in flames Thursday morning, leaving behind piles of charred parcel.

It's not clear what caused the fire at the Cargo Force facility at 84 Harbor Drive in Jersey City just after 11 a.m. The blaze was contained to the three trailers, and no one was hurt, according to the mayor's office.

Cargo Force is a transportation contract carrier for the U.S. Postal Service.

The barrel and stock of a long-arm firearm was found in one of the parcels; it appears it was being legally shipped, according to officials.

It's not clear what type of packages the trailers were holding, or where they were headed. A USPS spokesman says the investigation is in its early stages.

The U.S. Postal Police and the federal bureau of Alcohol, Tobacco and Firearms responded to the scene and are working with Jersey City fire investigators to determine the cause and origin of the fire.