GREENDALE, Wis. (AP) _ A construction accident Tuesday ignited a natural gas explosion that crumbled an apartment building for senior citizens, killing three residents and injuring six others, authorities said.

Fire Chief Arnold Heling said construction workers rushed to the aid of residents in the eight-unit apartment building after they struck the gas line, touching off a series of blasts.

The force catapulted at least one person from the building, Heling said. The bodies of three residents were pulled from rubble and six people, including five construction workers, were taken to hospitals for treatment of minor burns and other injuries, authorities said.

The Milwaukee County Medical Examiner's office identified two of the victims as Betty and George Heldak, both 78. The identity of the third victim, a man, was not immediately released.

Witness said a series of explosions beginning about 8 a.m. leveled about half the apartment building, which was used mainly to house senior citizens in Greendale, a suburb southwest of Milwaukee.

Frank Kritter, 44, said fire engulfed the complex within a few minutes of the blast and some elderly residents became trapped.

''First, I heard the contractor digging. Then I heard a backfire, a small explosion,'' he said. ''I looked out the window and saw it blow up. It happened so fast.''

Kritter said he and a neighbor rushed to the building and were able to help an elderly man escape before firefighters arrived. ''I saw one person at the window, but he didn't get out,'' he said. ''The smoke was billowing.''

Karen Fossum, a spokeswoman for Wisconsin Natural Gas Co., said the explosion at the complex was caused when a contractor working on the building's foundation struck a natural gas line.

''There was a contractor at the site doing a foundation repair to the basement and he hooked the gas line with a tractor and we believe that some sparks from the tractor ignited the natural gas line,'' Ms. Fossum said.

Heling said firefighters arriving at the scene found injured residents and McCoy Contractors' workers scattered about the site.

''A couple were blown out of the building,'' he said.

The injured, five men ages 26 to 58 and one woman age 86, were taken to the Milwaukee County Medical Complex in Wauwatosa but none suffered life- threatening injuries, hospital spokeswoman Jeanne Hahn said.

Ms. Fossum said gas was shut off to the complex after the explosion.

Don McCoy, owner of McCoy Contractors that was repairing water drains in the apartment complex, expressed remorse at the accident but praised his crew for attempting to rescue residents.

''I think what they did was very heroic,'' McCoy said.

Red Cross worker Nadine Sydow said residents told the agency two of the building's eight units were vacant and the occupants of another were vacationing.

Pauline Kallas, manager of the Greendale Golden Agers club that serves elderly residents at the complex, said she arrived on the scene immediately after the explosion.

''I saw a girl in her nightgown, lying in the snowbank,'' she said. ''It's a terrible thing.''