The Justice Ministry had requested US and French assistance for the inquiry into the death of Kassir, the An-Nahar columnist killed by a bomb that exploded under his car on Thursday morning.
Five foreigners, two in white overalls, were seen collecting evidence at the scene of the bombing in Beirut on Friday.
A Lebanese official at the site said the foreigners were from FBI.
Kassir, 45, was a university professor and founding member of the Democratic Left Movement, a small group that joined the anti-Syrian opposition and played an active role in the protest campaign against Damascus's control.
He wrote a column in An-Nahar, a leading newspaper that frequently criticises Syria, and was a regular on TV talk shows.
In a recent television appearance, Kassir said he had long received threats from security agents trying to silence him.
Samir Kassir was a vocal critic of
Syrian presence in Lebanon
An official at the US Embassy declined to comment on the report that FBI agents were assisting the investigation.
But in a statement on Friday, the embassy reiterated its condemnation of Kassir's killing and added: "The United States stands ready to respond rapidly to any requests for assistance."
The embassy hinted at the likely perpetrators, saying: "Kassir's enemies were those who participated in Syria's occupation of Lebanon, and his friends were those who dreamed of a free, democratic, peaceful and prosperous Lebanon."
The United States on Friday also urged the UN Security Council to expand its investigation into the slaying of former Lebanese Prime Minister Rafik al-Hariri to include the killing of Kassir, White House spokesman Scott McClellan said.
"We strongly condemn the brutal assassination of Mr Kassir," McClellan said.
"He was a prominent journalist who long advocated Syria's withdrawal from Lebanon. This heinous act was clearly an attempt to intimidate the Lebanese people and undermine their efforts to build a free and democratic future. I think it reflects an environment of political repression created by Syria's long military and intelligence presence inside Lebanon."
The February assassination of al-Hariri, a vocal opponent of Syrian influence in Lebanon, sparked anti-Syrian protests and international pressure that forced Syria to withdraw its army after 29 years of political and military dominance.
The opposition has blamed Syria and its Lebanese allies in the security services for al-Hariri's assassination, an accusation Damascus has denied.