In a statement, the top American diplomat yesterday said the "assassination" of Chatah - an influential member of a coalition opposed to Syria's ruling regime - represented a "terrible loss for Lebanon, the Lebanese people and for the United States."
Kerry forged a relationship with Chatah during several visits to Beirut during his long tenure as a US senator.
"I know he was a voice of reason, responsibility and moderation," Kerry said in a statement.
Kerry said the United States government would back Lebanon as it sought to hunt down "those responsible for this heinous and cowardly attack."
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Kerry's statement made no reference to the conflict raging across Lebanon's border in Syria, nor to Hezbollah's support of the Syrian regime, stating only that Washington wanted to see Lebanon maintain "its sovereignty and stability."
Chatah, a close adviser to former Lebanese Prime Minister Saad Hariri and an outspoken critic of Syria's ruling regime and Hezbollah, was killed in a huge bomb blast in the capital.