US President Donald Trump called Sri Lanka's Prime Minister Ranil Wickremesinghe on Monday to express condolences over the killing of nearly 300 people in suicide bomb blasts at churches and hotels.
The attacks on Easter Sunday, which Sri Lankan authorities believe were carried out by a local Islamist group, were "one of the deadliest terrorist events since the September 11, 2001 attacks on the United States," White House spokesman Hogan Gidley said.
At least four Americans were killed in the attacks, including a fifth grader (around 10 years old) who attended the private Sidwell Friends school in Washington.
"Prime Minister Wickremesinghe expressed appreciation for the president's concern and updated him on the progress of the investigation," according to the White House.
Trump told the premier that he "pledged United States support to Sri Lanka in bringing the perpetrators to justice, and the leaders reaffirmed their commitment to the fight against global terrorism."
Washington has pledged investigative assistance and is sending FBI agents to help support Sri Lankan police in the probe, The Washington Post reported.
It said the FBI has also offered laboratory expertise for testing bomb evidence, while analysts are reviewing bureau databases for any useful information.
In a brief statement on Twitter, Trump said he had also spoken with Sri Lankan President Maithripala Sirisena on Sunday.
He later deleted this statement and the White House could not be immediately reached for comment on why the president did so.
A State Department official said that at least four Americans were killed in the bombings.