A US federal prosecutor has charged three Sri Lankans in a Los Angeles court in connection with the 2019 Easter Sunday terrorist attack on the island nation that killed 268 people, including five Americans.

The Justice Department said on Friday that the three men, who are members of the ISIS in Sri Lanka, have been charged with “conspiring to provide, providing, and attempting to provide material support” to a foreign terrorist organisation.

ISIS in Sri Lanka is the arm of the dreaded Islamic State in Iraq and Syria terrorist group.

The three are in Sri Lankan custody and were identified by the Justice Department as Mohamed Naufar, the “Second Emir” of the ISIS in Sri Lanka; Mohamed Anwar Mohamed Riskan, who allegedly helped make explosives for the attacks, and Ahamed Milhan Hayathu Moahmed, who allegedly killed a police officer and shot a suspected informant.

The charges were filed on December 11 in the federal court in Los Angeles but announced only now by the Justice Department in the last days of President Donald Trump's administration.

The Department said that they were charged after a nearly two-year investigation by the Federal Bureau of Investigation (FBI), which assisted Sri Lankan authorities.

Assistant Attorney General for National Security John Demers said, “These charges reflect that the US justice system remains a powerful tool to bring to bear against those who harm our citizens abroad. We will continue to pursue justice for the victims of these heinous attacks and for all American victims of terrorism.”

Commerce Department official Chelsea Decaminada, who was in Sri Lanka on official assignment, was killed in the attacks that targeted Christian places of worship and hotels where foreigners stay.

Another victim was Kieran Shafritz de Zoysa, a fifth-grade student from a Washington school.

“The domestic charges announced today for an attack on foreign soil represent the FBI’s commitment to deliver justice to travelling American victims and to protect U.S. interests here and abroad,” said Kristi K Johnson, the FBI's Assistant Director in Charge of the Los Angeles Field Office.

ISIS leader Abu Bakr al-Baghdadi claimed responsibility for the terrorist attacks in Sri Lanka saying that they were in retaliation against the West for defeating his terrorist organisation in Baghuz, Syria, in March 2019.

ISIS was almost completely wiped out of territories it had held in Syria and al-Baaghdadi was killed in a US airstrike in October 2019.

Demers said that the charges against the three related to recruiting others to ISIS, purchasing and materials for and making explosives, training those who participated in the attacks, and murdering “in the name of this deadly foreign terrorist organization.”