A US anti-terrorism team has joined local police investigating the latest bombing to shake Lebanon.
The scene of the blast has been left for the Americans to inspect
Five FBI officers examined the site of Sunday's attack on TV journalist May Chidiac in Jounieh, north of Beirut.
Correspondents say it is the first time US agents have been openly called in by the Lebanese authorities and will raise pressure on neighbouring Syria.
Beirut admits failing to solve a series of deadly blasts against anti-Syrian figures, including ex-PM Rafik Hariri.
After talks with security officials this week, Prime Minister Fouad Siniora requested US and French security expertise to help halt the bomb attacks.
The Federal Bureau of Investigations agents inspected the site of Sunday's blast near the Christian port of Jounieh.
Popular anchorwoman May Chidiac lost a leg and an arm in the attack, which has provoked widespread public indignation.
The agents sifted through the debris, collected fragments and took photographs of Ms Chidiac's wrecked Range Rover.
The attack on May Chidiac was condemned in all quarters
Journalists were kept behind a police cordon and the Americans did not respond to questions shouted to them.
A black box carried by one member of the team was marked "explosives unit" and gave the FBI's Washington address.
Armed Lebanese police officers guarded the site as the inspection proceeded.
The US has recently stepped up warnings to Damascus to stop interfering in Lebanese affairs.
A parallel investigation by a UN appointed German prosecutor returned to Lebanon from Damascus last week after questioning Syrian officials.
Detlev Mehlis is expected to issue his report on the killing of Rafik Hariri at the end of October.
Correspondents say he could possibly implicate Syria and loyalist elements in Lebanon.
After the 14 February assassination, Syria bowed to international pressure by withdrawing thousands of its troops and intelligence agents whom it had used to control Lebanese politics since the Civil War.
There have been at least 13 bomb attacks since February, some of which have killed prominent critics of Damascus and its supporters in Lebanon.