"This was a calculated attack with enormous political implications since the entire leadership of the Awami League was present at the rally," the letter sent on August 17 said.

In the letter, the congressmen representing both Democrats and Republicans, categorically criticised the government for its failure to bring the perpetrators of the attack to justice and said the evidence was not preserved properly for investigation.

This was a high-level reaction from the US side on the eve of second anniversary of the deadly attack on AL rally, which was an attempt to assassinate AL chief Sheikh Hasina and other senior party leaders.

"Despite assurances of a proper investigation of the attack, evidence was not properly preserved and individuals with no connection to the crime were brought forward as false suspects and ultimately released," they said.

They further observed that those responsible appear to have gotten away with impunity.

"Besides, assistance provided at the request of your (Khaleda Zia) government by the United States and other nations was not utilised fully to investigate the attack," the letter said.

The attack subsequently escalated political violence in Bangladesh in both quantity and severity that included killing of former finance minister and AL leader Shah AMS Kibria, emergence of Islamic radical groups, the first suicide bomb attacks in the history of Bangladesh, along with 469 simultaneous bombings across the country in last August, the congressmen said.

Failure of the investigation is symbolic of a serious diminution of rule of law in Bangladesh and it has emboldened lawlessness and allowed Islamic extremist elements to gain ground in Bangladesh, they added.

They also expressed concern over the impact of the failure of the investigation into the attack on the next general elections in Bangladesh.

"With the national election in less than six months the failure to apprehend suspects and mete out justice in this high profile attack on the head of the opposition party, send a disconcerting message to voters for the potential of intimidation and violence in the run up to the election."

The signatories of the letter are Joseph Crowley, Charles Gonzalez, Alcee Hastings, Frank Pallone, Diane E Watson, Eliot L Engel, Robert Wexler, GK Butterfield, Corinne Brown, Janice D Schakowsky, Michael Capuano, Barney Frank, Brian Higgins, Sherrod Brown, Debbie Wasserman Schultz, Kendrick B Meek, Anthony D Weiner and Al Green.