Hannan Khan headed back to Dhaka on June 22 after two weeks of investigation there.
On Monday, the second war crimes tribunal of Bangladesh indicted Ashrafuzzaman Khan.
Justice Obaidul Hassan-led ICT-2, formed to expedite trials of crimes committed during the nation's struggle for freedom in 1971, ordered the trials to commence in absentia from July 15.
Ashrafuzzaman is reportedly living in Queens, in New York.
Before leaving the US, investigator Hannan Khan told bdnews24.com he had visited Ashrafuzzaman-run ICNA (Islamic Circle of North America) at Queens.
He alleged the collaborators of Pakistan army were using several organisations in the US, including ICNA, to confuse the Muslim community there.
Ashrafuzzaman, hailing from Gopalganj's Bezra Bhatra, has been charged with killing journalists, eminent writers, teachers, doctors, and intellectuals during the war.
He had allegedly led the notorious ‘Al Badr’, which spearheaded execution of the Bengali intelligentsia during the 1971 Liberation War and said to be its ‘chief executor’.
He is also said to have been an active member of Islami Chhatra Sangha – the student affiliate of Jamaat-e-Islami in 1971. The Jamaat sided with Pakistan and opposed the independence of Bangladesh.
Investigation officer Hannan Khan also sought information about former Pirozpur Mathbaria MP Abdul Jabbar.
Jabbar, a former Jatiya Party leader, is currently believed to be in Bronx, New York.
Hannan Khan said he had spoken with various US quarters about the ongoing war crimes trials.
He had urged Bangladesh's permanent envoy to the UN AKA Momen to take steps to check the false and baseless propaganda to save the war criminals.
"The activities of the international crimes tribunals are going on with clarity," he said.