WASHINGTON -- Investigators are holding out little hope of capturing whoever fired a gun at least four times at the White House, piercing a State Dining Room window with one shot.


The Secret Service confirmed that one 9mm bullet was found Saturday in the State Dining Room -- one floor below the president's residence. Other bullets were found on the landing of a first-floor balcony, a driveway outside the back door and a nearby Christmas tree.


President Bill Clinton and his family were sleeping in their top-floor residence during the shooting early Saturday, and were never in danger, the Secret Service said.


But the late discovery of a bullet inside the mansion made the third White House security breach this year especially disturbing. A suspect who peppered the White House with bullets in October managed to hit a press room window in the West Wing.


Although some White House windows are not bulletproofed, the president is not at risk if a bullet is fired at a window, law enforcement officials said. Speaking on condition of anonymity, they said bulletproof panels are wheeled in front of the windows when the president or other dignitaries are in the room. The State Dining Room was not in use during the early-morning shooting.


With their crime scene search completed, investigators said Sunday they believed all bullets on the White House grounds had been found. Secret Service agents reported hearing just four shots, although other witnesses said they heard six.


A preliminary ballistics examination found that all the bullets came from the same weapon, according to a Secret Service statement. Additional tests to determine the type of weapon will be conducted, they said.


Nine-millimeter ammunition can be fired from handguns, shoulder-strap weapons and some rifles. The distance a bullet can travel depends on the type of weapon used.


The bullets found were in good condition, indicating that they were fired from a great distance and had not struck with enough force to be greatly disfigured. The dining room bullet pierced the glass "and just fell near the window. Plop," one official said on condition of anonymity.


That could suggest the White House was not the target. Several officials, including Chief of Staff Leon Panetta, speculated that the incident was a drive-by shooting in a city besieged by gun violence.


A public street loops behind the White House grounds less than 100 meters from where the bullets were found.


But investigators also noted that the bullets were found within 100 meters of each other, suggesting the gunman took aim at the White House.


Bullets found outdoors in a Christmas tree adjacent to the diplomatic entrance, and on a nearby driveway, struck places where Clinton routinely walks.


Investigators said there were few clues in the case and no description of a suspect.


Less than two months ago, a gunman sprayed the front of the White House with semiautomatic weapon fire while Clinton was upstairs watching TV. A 26-year-old Colorado man, Francisco Martin Duran, has been charged with attempted assassination in that incident and is awaiting trial. Officials say there was no connection between the two shootings.


In September, a small plane crashed-landed on the south grounds of the White House and its pilot was killed.