The letter comes one month after President Donald Trump praised Haftar during a phone call as the field marshal's troops continued their offensive against the UN and US-supported government in Tripoli.
"A United States citizen is directly undermining United States policy in Libya, including US support for a United Nations-led mediation process and the internationally recognized government of Libya, at the same time, Mr. Haftar's forces are alleged to have committed crimes and inflicted unnecessary suffering and cruelty during the course of military operations, "the letter reads.
The request sent to the Justice Department was signed by five House Democrats – Tom Malinowski of New Jersey, Gerald Connolly of Virginia, David Trone of Maryland, Ted Lieu of California and Colin Allred of Texas ̵
seize Tripoli from the United Nations-backed Government of National Accord in April
On Wednesday, Amnesty International revealed new evidence of war crimes in Libya, including indiscriminate attacks on civilian residential areas.
Amnesty said satellite images and testimony indicated that "densely populated residential areas in the Abu Salim district of Tripoli were indiscriminately attacked with rockets during an episode of intense fighting between April 15-17."
Haftar's forces and a local militia affiliated with the Government of National Accord blamed each other for the indiscriminate attacks in southern Tripoli. Residents interviewed by Amnesty told the nongovernmental organization that they believed Haftar's forces were responsible
Libya has been plagued by violence since the oyster and killing of longtime strongman Moammar Gadhafi in 2011.
More than 400 people have been killed in the latest offensive and more than 2,000 wounded, according to the World Health Organization. The UN's humanitarian affairs office said more than 60,000 people have fled their homes.
Trump was strongly criticized for praising Haftar last month as his forces moved in on the capital. The White House's official readout via a phone call between Trump and Haftar made no mention of the offensive on Tripoli, which Secretary of State Mike Pompeo had criticized earlier in April.
Even after that call, Trump has not picked a page in the Libya conflict, a source familiar with internal White House discussions about the issue customs CNN, adding that the President agreed to speak with the request of Egyptian leader Abdel Fattah el-Sisi.
Trump had with Sisi at the White House in early April.