John Elijah Settles, charged with second-degree murder in the shooting death of his nephew, 12-year-old Mark Settles, was ordered held in lieu of $3,000 bond yesterday in D.C. Superior Court and to undergo a medical exam to remove evidence -- a bullet from what police say is a self-inflicted gunshot wound -- from his left leg.

Assistant U.S. Attorney Debra Long-Doyle had asked Commissioner John King to set Settles' bond at $50,000. His background of emotional problems and narcotics-related convictions, and his two versions of how Mark's death occurred were cause to believe he could not be relied upon to appear in court on his own volition, she said.

"The fact that he gave conflicting versions to police shows his instability," Long-Doyle said.

In addition, she said, Settles has threatened to kill himself since police arrested him Tuesday at St. Elizabeths Hospital, where he had checked in last weekend for drug abuse treatment.

"The police were concerned because of his statements that he wanted to kill himself," Long-Doyle said. She said Settles had been placed under a suicide watch.

Settles, 24, is charged with killing his nephew on Dec. 17 in the family's Stoddert Terrace apartment in Southeast Washington. The family dog also was shot and killed, and police say John Settles received a bullet wound in the left leg in the incident.

Ballistics tests have shown that the bullets found in Mark and the dog, Old Cyrus, match the .22-caliber gun found near John Settles. The bullet lodged in his leg is believed to have come from the same gun and is considered evidence in the case. In court yesterday, King ordered Settles not to tamper with, disturb or try to dislodge the bullet.

In arguing for Settles' release to a family member yesterday, public defender Samuel Delgado said that the ballistics evidence presented in the Settles arrest affidavit was biased because it came from a branch of the police department. He asked King to allow Settles to be released to a sister who was present in court. Eleanora Settles, John Settles' sister and Mark's mother, was not present.

Settles has given police two different accounts of the shooting, which occurred on a day that Mark, a sixth grader, was home from school with an eye infection. Mark's mother had left the house to visit a relative who lives nearby.

In his Dec. 31 version, according to police affidavits, Settles told police that he heard a knock on the door and told Mark to answer it. Seconds later, he got a gun, which he had recently purchased, from the bedroom. He moved into the hallway toward Mark and was shot. He told police he did not see who shot him and that he collapsed.

In the version he gave to police Tuesday, Settles claimed that the shootings occurred during an argument in which he was advising Mark to stay away from drug activity on the streets. Settles told police that he hit Mark and that Mark pulled a gun. In the ensuing struggle, both were shot.

Although police have raised questions about Settles' accounts of the shooting, they say they have obtained independent information from people in the Settles' neighborhood that Mark was involved in the area's drug trade, a claim heatedly disputed by the boy's family, neighbors and youths on the street who claim to be drug dealers.

John Settles recently completed a period of probation after a 1986 conviction for possession of PCP and distribution of marijuana, Long-Doyle said. In 1984, he was convicted of grand larceny.