Washington once labelled Khartoum a hub of the "axis of evil".
White House says Sudan's transitional government has deposited $335 million for compensating survivors and family members.
President Muhammadu Buhari has not yet directly commented on the shooting, drawing fierce criticism for his silence.
Police said assailants had attacked the site on the second day of violence in the city of 20 million people.
Secretary of State Mike Pompeo says the US welcomes an immediate investigation into any use of excessive force.