Authorities have cordoned off the site of Tuesday's blast in Karantina north of Beirut which killed three passers-by and wounded 26 others.
A giant white tent was placed over the scene to preserve evidence.
On Thursday, U.S. investigators began a "technical survey of the scene in the presence of Lebanese forensic experts," a senior Lebanese security official said.
He said the team included FBI agents who were flown in Wednesday.
Associated Press Television News footage filmed from a distance showed at least three people who appeared to be U.S. experts, one of them wearing white overalls and another dressed in dark blue.
They were seen collecting evidence from the wreckage of several vehicles that were damaged in Tuesday's blast, including the armor-plated U.S. Embassy SUV -- the bomb's target.
The FBI has also aided in the investigations into several mysterious bombings against anti-Syrian figures in Lebanon over the past three years.
In Washington, State Department spokesman Sean McCormack had said that a joint team of agents from the department's Bureau of Diplomatic Security and the FBI would travel from Washington to Lebanon on Wednesday to investigate the incident.
The embassy, about 13 kilometers (8 miles) from the explosion site, told Americans Wednesday to avoid popular public places in case of further attacks, and departing U.S. Ambassador Jeffrey Feltman canceled a meeting and dinner in Beirut.
Tuesday's bombing occurred shortly before the embassy was to hold a farewell reception in a Beirut hotel for Feltman. That reception was canceled.
But the ambassador on Thursday made a farewell visit to Parliament Speaker Nabih Berri. He repeated that the attack would not weaken Washington's commitment to democracy in Lebanon.
"This attack will certainly not weaken the international resolve for Lebanon. It will not undermine U.S.-Lebanese partnership," he told reporters.
The attack -- which coincided with U.S. President George Bush's Mideast tour -- highlighted the growing chaos in Lebanon, which has descended into violence over the last three years after almost a decade of calm following its long civil war.
U.S. diplomats are deeply involved in the country's fractured politics, supporting the government against the Syrian-backed opposition. A series of bombings and political assassinations targeting mostly anti-Syrian politicians and journalists have plagued the country since 2005.
McCormack said it was still unclear who was behind the attack, but stressed that the U.S. would not back down on its support for Lebanon as a result.
The embassy advisory on Wednesday reminded all Americans in Lebanon "to maintain a high level of vigilance, especially when planning travel." It added: "Americans are also advised to avoid popular gathering spots."(AP-Naharnet)