NUEVO PROGRESO, Mexico -- A propane tank explosion ripped through a restaurant Saturday morning at the Mexican border with Texas, killing at least six people and injuring several more.

The tank exploded at about 8:30 a.m. at the Red Snapper restaurant, located about two blocks from a bridge over the Rio Bravo connecting Mexico with the United States.

The three-story cement building and a neighboring pharmacy and souvenir shop were demolished in the blast, which shattered windows up and down the street. Investigators said the explosion appeared to be accidental.

The six people killed were Mexican citizens, Mexican police officer Raul Herrera Soto said. Four injured people were hospitalized; their conditions were not immediately available.

The restaurant was not open when the tank exploded on the second floor, but employees were in the building, said George Garrett of the Weslaco, Texas, emergency management team.

It was unclear how many people were trapped in the rubble.

A four block area was cordoned off by authorities, and the McAllen, Texas, Fire Department used rescue dogs and thermal imaging equipment to search for survivors.

"It's kind of like a puzzle," said Ramon Garcia, a judge in neighboring Hidalgo County, Texas, who was helping coordinate U.S. emergency crews. "We need to be careful, otherwise we're going to be hurting people who are trapped inside."

The bridge from Mexico to Progreso, Texas, was closed because of the explosion, though rescue workers from the United States were allowed to enter Mexico.

The city along the Rio Bravo river -- called the Rio Grande in the United States -- is a popular destination for tourists from South Texas who flock to the area's restaurants, bars and shops.

Witnesses said the ground shook and a thick cloud of dust filled the air.

"It was a loud boom and we heard the echoes. It kept echoing," said Jose Ramirez, who owns a fabric shop across the street and heard the explosion from his home four blocks away.

Crowds still were sparse at the time because of the early hour, local police spokesman Enrique Hernandez said.

"Normally, there's a big flow of people," Hernandez said. "Maybe that's why more people were not injured. We hope we don't locate any more bodies."