U.S. Ambassador Francis Ricciardione told reporters that a guard at the gate was killed in the 1:15 p.m. blast, and a Turkish citizen was wounded. Reuters quoted Ricciardione as saying the slain guard was a Turkish national, but that information could not be immediately confirmed. U.S. Embassies are usually guarded by a combination of local security personnel and American diplomatic security forces.
The bomb appeared to have exploded inside the security checkpoint at the side entrance of the embassy, but did not do damage inside the embassy itself. Footage showed that the door had been blown off its hinges and debris littered the ground and across the road. An Associated Press journalist saw a body in the street in front of an embassy side entrance.
Police swarmed the area and several ambulances were dispatched. An AP journalist saw one woman who appeared to be seriously injured being carried into an ambulance.
The police official, who spoke on condition of anonymity in line with government rules, said police had examined security cameras around the embassy and had identified two people who could have been the suicide bomber.
The phones were not being answered at the embassy. "The US Embassy would like to thank the Turkish Government, the media, and members of the public for their expressions of solidarity and outrage over the incident," it said in a statement.
The embassy building is heavily protected. It is near an area where several other embassies are located, including that of Germany and France. Police sealed off the area and journalists were being kept away.
There was no claim of responsibility, but Kurdish rebels and Islamic militants are active in Turkey. Kurdish rebels, who are fighting for autonomy in the Kurdish-dominated southeast, have dramatically stepped up attacks in Turkey over the last year.
As well, homegrown Islamic militants tied to al Qaeda have carried out suicide bombings in Istanbul, killing 58, in 2003. The targets were the British consulate, a British bank and two synagogues.
In 2008, an attack blamed on al Qaeda-affiliated militants outside the U.S. Consulate in Istanbul left three assailants and three policemen dead.
In the November 2003 attack on the British consulate, a suspected Islamic militant rammed an explosive-laden pickup truck into the main gate, killing British Consul-General, Roger Short, and his assistant, Lisa Hallworth.
CBS News senior correspondent John Miller, a former deputy director in the National Intelligence Director's office, says al Qaeda will be one of the first groups considered as suspect in the attack, but he also notes the recent Israeli airstrike in Syria, for which Damascus and its staunch ally Iran vowed to retaliate.
Iran, Miller says, has extensive intelligence operations in Turkey, and its Islamic militant group ally, Hezbollah, also has significant assets there.
"Iran, in terms of retribution, would stack Israel and the U.S. together," says Miller.
Turkey has become a harsh critic of the regime in Syria, where a vicious civil war has left at least 60,000 people dead. The first of six Patriot missile batteries being deployed to Turkey to protect against attack from Syria was declared operational and placed under NATO command on Saturday and others were expected to be operational in the coming days.