KANSAS CITY, Mo. -- Federal and local authorities on Tuesday were investigating an explosion and two-alarm fire that ripped a popular Kansas City restaurant.

Battalion Chief Joe Vitale said two passersby reported an explosion in the Hereford House at East 20th and Main streets at about 12:30 a.m. Monday.

The blast blew a sizable hole in the brick veneer on the building. Flames raced through the structure, sending thick, black smoke over the neighborhood.

Vitale said damage was estimated at more than $1 million.

No injuries were reported, and no one was believed to have been inside the restaurant at the time.

Officials from the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives, the police bomb and arson squad and fire department were investigating.

A team of 20 investigators from the ATF spent most of Tuesday making sure the structure was secure enough for them to go inside to investigate during the next couple of days.

"The simple answer is we're looking for things that shouldn't be there," one agent, Kenneth Chisholm, said.

Police cautioned that investigators had not determined that the fire was arson.

Owner Rod Anderson said he had no reason to believe anyone would target his business.

"Obviously the Fire Department called in people, but I'm hoping it was just a freak deal," Anderson said.

Callers to 911 described the explosion.

"At 20th and Main, Hereford House is on fire. It just exploded," one caller said.

Another said, "The side of the building just exploded."

When a dispatcher asked the caller what she saw, the caller said, "The side of the building just literally blew out. I mean, there's smoke piling out of it. There's flames."

The Hereford House first opened in 1957.