Lebanon said it prevented an attack on the heavily-fortified US complex on Wednesday, arresting a Lebanese man at the embassy gates who tried to enter with a bomb concealed in a sports bag.
His Palestinian companion was also arrested.
A security source told Aljazeera that the suspected ringleader, a Lebanese man called Mahdi al-Hajj Hasan, was under investigation over the explosion.
Another source told Reuters news agency that the man was arrested on Friday after authorities obtained his name from questioning the two men already in custody.
"He is supposed to be the leader, the one who sent the people to the US embassy," the source said.
He added that charges could be filed as early as Monday.
Earlier this week Lebanon charged 10 people with "terrorist" links over a string of bombings against American fast food outlets over the past two years and a plot to kill US ambassador Vincent Battle.
Anti-US attacks were common
during Lebanon's civil war
Some of those arrested are alleged to be linked to Usama bin Laden's al-Qaida network.
Lebanon was the scene of dozens of anti-US attacks in the 1980s with bombings against the US embassy, its annex, and the main Marine barracks in which hundreds were killed.
Several US nationals and other Westerners were also kidnapped.
The attacks abated after the end of the 1975-1990 Lebanese civil war. But a series of much smaller attacks have targeted mainly US and British interests since the eruption of the Palestinian uprising against Israeli occupation, and the US-led war on Iraq.