The U.S. State Department says "preliminary evidence" suggests a U.S. embassy car was the target of Tuesday's blast that killed three people in the Lebanese capital, Beirut.
A spokesman, Sean McCormack, says a joint State Department-FBI team is going to Lebanon to investigate the blast, which struck the U.S. embassy vehicle and wounded its Lebanese driver.
Lebanese security officials say the three fatalities were Lebanese and Syrian bystanders.
Security was tight around Beirut Wednesday, as Lebanese troops set up checkpoints and investigators gathered evidence around the bomb site. The U.S. Embassy in Beirut restricted movement of its staff.
Lebanese security officials say a stolen car packed with 20 kilograms of explosives appears to have caused the blast, in a northern suburb of the capital city.
No group has claimed responsibility.
U.S. Secretary of State Condoleezza Rice called the blast a terrorist act. She said the United States will not be deterred from its efforts to promote democracy in Lebanon. The country is in a political crisis, having been without a president since November.
A series of bombings in Lebanon in recent years mainly has targeted prominent anti-Syrian politicians. Syria has denied involvement in any of the attacks.
Last month, a car bombing killed Lebanese Brigadier General Francois al-Hajj on the outskirts of Beirut. He played a key role in the army offensive against al-Qaida-inspired militants at a Palestinian refugee camp, Nahr al-Bared, last year.
Some information for this report was provided by AFP, AP and Reuters.