The assistance was offered as Secretary of State John Kerry called Bangladesh Prime Minister Sheikh Hasina yesterday.
"The Secretary encouraged the government of Bangladesh to conduct its investigation in accordance with the highest international standards and offered immediate assistance from US law enforcement, including the FBI," State Department spokesperson John Kirby said.
The offer came in the wake of the "outrageous attack" on the Holey Artisan restaurant in Dhaka's high-security diplomatic zone, he said.
"He re-affirmed US support for Bangladesh's efforts to bring those responsible for planning and conducting the attacks to justice as well as to prevent future attacks," Kirby said.
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Twenty hostages, including Indian girl Tarishi Jain, were hacked to death by terrorists inside the cafe popular with expats in the diplomatic zone before commandos launched an assault, killing six attackers and capturing one alive. Two police officers were also killed in the attack.
Most of those killed were found with their throats slit.
Bangladesh, however, blamed "homegrown" Islamist terrorists and Pakistan's spy agency ISI for the attack, ruling out involvement of the Islamic State.