The letter is from Rep. Ted Lieu (D-Calif.), a former Air Force lawyer and member of the House Foreign Affairs Committee.
His past statements about women are at issue.
The strike is reminiscent of some the worst days for civilians in Afghanistan and the Gulf War.
The March 19 attack, probably carried out by a drone, killed Qari Yasin in Afghanistan's Paktika province.
The U.S.-led coalition previously said that officials were unsure whether it had conducted air attacks targeting the affected site.
In the last week three mass casualty incidents have been attributed to U.S.-led forces in Iraq and Syria, handily making March one of the most lethal months for civilians in the the two-year-old war against the Islamic State
"The goal is to steal the fear away from the black flag of ISIS," said former Green Beret Loren Schofield.
The Army has no record of retired Lt. Gen. Michael Flynn asking for permission to take any kind of 'foreign employment.'
The news comes just as an iconic district in the country's south falls to the Taliban.
The operation began Tuesday night and focused around the area of the Tabqa dam--a choke point on the Euphrates river that has been a key hub for the Islamic State.