WASHINGTON (Reuters) - Federal investigators searched on Saturday for a motive behind a lone gunman‘s attack on the U.S. Capitol that left two police officers dead. President Bill Clinton called it a „moment of savagery at the front door of American civilisation“ and lawmakers paid emotional tributes to the dead officers. The suspected killer, Russell Eugene Weston Jr., 41, faced charges that could get him the death penalty. Weston was unconscious in critical condition at a local hospital under heavy police guard and was given a 50-50 chance of surviving his multiple gunshot wounds. He had not been able to speak to investigators. The office of Republican whip Tom DeLay said Monday‘s House session would be devoted to tributes to the two dead officers, John Gibson, 42, and Jacob „J.J.“ Chestnut, 58. Weston‘s father issued a statement through a pastor apologising to the nation as a whole for the „trauma our son has caused.“ Lawmakers were determined to keep the Capitol, a venerable landmark for many Americans and foreigners as the seat of the U.S. parliament, open to the public. Tourists were permitted on Saturday to tour all but the crime scene area on the East Front. The Federal Bureau of Investigation searched Weston‘s cabin in rural Montana and his family home in Illinois while prosecutors in Washington charged him with murder of a federal officer. Killing a federal police officer carries a maximum sentence of the death penalty. Gibson and Chestnut were the first officers from the Capitol Police to be killed in the line of duty. Lawmakers wiped tears from their eyes as they praised them for laying down their lives to protect others. At the Capitol, flags flew at half-staff, bouquets of lilies, carnations and roses were placed on the steps and Capitol Police wore black bands over their badges. A 24-year-old tourist injured in the cross-fire, Angela Dickerson, was treated at George Washington University Hospital and released, hospital spokeswoman Lisa Saisselin said. Her family requested privacy and no details of her condition were made public. Weston allegedly stormed through the Document Door entrance, usually reserved for lawmakers and their staff, at 3:40 p.m. (1940 GMT) and began shooting. He was known to federal intelligence agencies and was considered a „low-level threat“ to Clinton by the Secret Service. Asked about Weston‘s mental health, a law enforcement official said, „He‘s the only one who knows. Apparently, he‘s a nut.“ In Valmeyer, Illinois, the Rev. Robin Keating, pastor of Zoar United Christian Church, read a statement from Russell Weston Sr. that spoke of „great sorrow,“ sorrow for the dead officers‘ families and „sorrow for the children that lost their daddies.“ „Our apologies to the nation as a whole, for the trauma our son has caused. To say that we are sorry is very inadequate. If I had but one wish, it would be that this had never happened.“ Monroe County Sheriff Dan Kelley, speaking to reporters near the family home, said there was no sign Weston had belonged to a militia group, describing him as a „loner.“ „There‘s no indication he had any connection to a militia group,“ he said, adding he had no criminal record in the state. In Montana, officials said Weston had been involuntarily committed to a mental hospital and claimed to have „dirt“ on President Clinton. A memorial service for the dead officers was planned for next week. Both the officers had 18 years service; Gibson had three children and Chestnut, five. Gibson was the plainclothes officer assigned to provide security for Republican Whip DeLay a little over three years ago. He died in DeLay‘s office suite. DeLay told reporters he had become very close to Gibson and felt responsible to ensure families and widows of both officers were taken care of „for the rest of their lives.“ „John Gibson was a hero,“ he said. „John Gibson was one of the finest men I knew.“ In a televised address, House of Representatives Speaker Newt Gingrich called the dead men „genuine heroes.“ His eyes wet with tears, the Georgia Republican asked the nation to pray for their families: „Help them to remember the heroes they sacrificed for their country.“ Clinton, on his way to Norfolk to dedicate a new aircraft carrier, called the incident „a moment of savagery at the front door of American civilisation.“ „(They) laid down their lives for their friends, their co-workers, and their fellow citizens, those whom they were sworn to protect. In so doing, they saved many others from exposure to lethal violence,“ he said. According to the FBI affidavit filed in D.C. Superior Court, the gunman entered the East Front of the Capitol and shot Chestnut in the head. He then went around the magnetometer and ran down a hallway where he encountered Gibson and shot him in the chest. Gibson managed to return fire and Weston dropped. Capitol Police spokesman Dan Nichols said that after Chestnut was shot, the gunman turned down the first corridor and pushed through a door marked „Private, Do Not Enter“ which leads to a warren of offices used by DeLay. He was confronted by Gibson, who had already shouted to nearby staffers to take cover. The two men traded shots and both fell wounded, lying near an office photocopier. Another officer who ran to the scene straddled the bleeding gunman, pointing his weapon at him, an aide to DeLay said. Both wounded men were treated by Sen. Bill Frist, Republican of Tennessee and a heart surgeon, who doctors said went with Weston to the hospital and possibly saved his life. Other emergency medical personnel attended to Gibson. DeLay was immediately flown home to Texas because police were uncertain if he had been the target, but he returned to Washington overnight. Asked if he felt the gunman was looking for him he said, „I don‘t think so, I hope not.“ When the shots rang out, DeLay ran into the bathroom by his office and huddled there for 10 minutes with a couple of women until police told him it was safe. „It was quite a gruesome scene,“ DeLay said.