John Bolton, the US ambassador to the UN, also hinted at Syria's involvement, noting the string of killings of anti-Damascus politicians in Lebanon.
"[The killing] shows the vicious face of those who oppose freedom"
George Bush, US president
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"One can follow the logic there, I think," he told CNN.
The UN Security Council has authorised the setting-up of a tribunal to investigate the killings of al-Hariri and 14 others in a string of attacks.
Bolton said the US will seek to add Gemayel's murder to the list of those whose alleged killers would be prosecuted by the tribunal.
George Bush condemned the assassination and accused Syria and Iran of attempting to "foment instability and violence" in Lebanon.
The US president described the shooting as "the vicious face of those who oppose freedom" and vowed to support government of the president, Fouad Siniora.
A three-day period of national mourning has been officially announced in Lebanon, where independence day celebrations have been called off.
The killing has increased tensions at a time when Lebanon was already facing a worsening political crisis.
The Shia muslim group Hezbollah and its pro-Syrian allies have threatened massive protests - aimed at bringing down Siniora's government - unless it gives them greater power.
Demonstrations planned by Hezbollah for Thursday have been postponed.
The Lebanese government is dominated by opponents of Syria and many see the demands as a bid by Damascus to restore its influence in its smaller neighbour - and by Hezbollah to boost its power.