Osama bin laden, hunted as the mastermind behind the worst terrorist attack on US soil, has been killed, President Obama announced tonight. The president called the killing of bin Laden the "most significant achievement to date" in the effort to defeat al Qaeda. "Justice has been done," Obama said. Bin Laden was located at a compound in Abbottabad, Pakistan, which was monitored and when the time was determined to be right, the president said, he authorized a "targeted operation." "A small team of Americans carried out the operation," Obama said. "After a firefight, they killed Osama bin Laden and took custody of his body" DNA testing confirmed that it was bin Laden, sources told ABC News. Obama said tonight that he was briefed last August on a possible but "far from certain" lead to bin Laden, but it took many months for the intelligence community to "run this thread to ground." "I met repeatedly with my National Security team as we developed more information about the possibility that we had located Bin Laden hiding within a compound deep inside Pakistan," the president said. "Finally, last week I determined that we had enough intelligence to take action and authorized an operation to get Osama Bin Laden and bring him to justice," he said. Sources said the attack was carried out by Joint Special Operations Command forces working with the CIA. AP Photo This April 1998 file photo shows exiled al... View Full Size AP Photo This April 1998 file photo shows exiled al Qaida <b>...</b>