An 18-month-old Southeast Washington boy accidentally shot and critically wounded himself yesterday with a .22-caliber handgun he found hidden beneath the mattress of a bed in his baby sitter's home, D.C. police reported.

Police said the boy, Rasheed Jefferson, was being cared for at the home of Shirley Poston, 49, of 1618 T St. SE, when the shooting occurred shortly before 1 p.m. They said the boy lives about a block away from the Poston home with his mother.

The child was taken to Children's Hospital, where he was admitted in critical condition with a single gunshot wound to the abdomen, police said. He had gunpowder burns on both hands, police said, common in self-inflicted gunshot wounds.

Police said Poston, who baby-sits for three other children, told them she did not know how the boy might have found the handgun. The gun was unregistered, police said.

Poston said she heard the shot minutes after Rasheed and her 5-year-old grandson went upstairs, apparently to use the bathroom, police said. They said Poston told them she was in the kitchen, cooking lunch for the children and talking to two female visitors, when she heard the noise --like an exploding firecracker -- of the small handgun being fired. Within seconds, her grandson was running back down the steps, screaming, "Mama, Mama, Rasheed shot himself," according to police.

Poston ran upstairs, police said, and found Rasheed leaning over a bed in the spare bedroom, clutching his abdomen. She turned the boy around and saw that he was bleeding. Hospital officials reported that the bullet pierced the child's stomach, liver and lung, then exited through his back.

Police investigators on the scene said that they were not sure of the precise sequence of events because by the time they arrived, the gun had been put back beneath the mattress. Also, police said, adults in the house gave conflicting reports of the incident.

"Mrs. Poston said that the 5-year-old was on the steps after the gun went off, and the other people in the house said the boy was upstairs in another room," a detective said.

The Poston home, a green wood-and-brick row house with a sizeable front yard, is located on a quiet Anacostia street, but the surrounding area is "infested with drugs and crime. There are a lot of burglaries in the area," said Constant B. Pickett, a D.C. police officer who lives nearby and who said his 15-month-old son is cared for by Poston during the day.

Picket said he rushed over to the house when he heard the initial report of the shooting, fearing that his own son might have been involved. He said he did not know there was a handgun in the Poston home, but added, "I can understand them having protection."

"All the children in the neighborhood are crazy about them the Postons ," Pickett said. "I'm going to take my son right back over there tomorrow because she's a very good baby sitter. She spends time with them, she feeds them and looks after them as if they were hers."