Searchers used shovels, small bulldozers and infrared gear Tuesday as they tried to find four people believed trapped under tons of rubble by an explosion at a huge grain elevator complex.

Two people were killed and 11 injured Monday when the blast blew out both ends of the DeBruce Grain Elevator.Rescuers heard "thudding sounds" in the area they were searching early Tuesday, Fire Chief Gary Curmode said.

"We don't know if they're still alive or if they're dead," said Horacio Velasquez, whose 25-year-old brother-in-law, Noel Najeroa, was among the missing workers. "We're just waiting."

Crews were trying to shovel through loose grain to reach the workers, thought to be trapped about 800 feet down a 1,200-foot-long tunnel underneath the complex. By daybreak - almost 24 hours after the explosion - they were still an estimated 400 to 450 feet away as grain kept cascading into their way, said Curtis Wilson of the Sedgwick County Fire Department.

The rescuers tried to shore up the tunnel with timber and plywood, and used ultrasound and dogs in addition to infrared equipment to try to detect the missing men. Workers also searched a parallel tunnel, where the loose grain wasn't as much of a problem. Four 1,200-foot tunnels are under the complex.

The explosion blew out both ends of the elevator, knocking holes the size of a house in the 165-foot-tall headhouse. That centrally located portion of the complex, about 35 feet taller than the bins, houses the main conveyer system and the control center from which grain movement is regulated.