CHICAGO — The state-appointed emergency manager of the Detroit Public Schools, who had also managed the city of Flint and oversaw its decision to draw its water from the Flint River, resigned on Tuesday, the day before a congressional committee hearing in Washington on Flint’s water.

Michelle Zdrodowski, a spokeswoman for Detroit Public Schools, said the manager, Darnell Earley, had declined to testify before a panel investigating the widespread contamination of Flint’s tap water after the city switched its water source, which led to elevated blood lead levels throughout the city.

On Tuesday evening, the House Oversight and Government Reform Committee issued a subpoena for Mr. Earley to testify on Wednesday, but his lawyer, A. Scott Bolden, said Mr. Earley would not be able to appear.

Representative Elijah E. Cummings of Maryland, the ranking Democrat on the committee, said in a statement, “Mr. Earley has a right to assert the Fifth Amendment, but his abrupt resignation earlier today and his refusal to testify before the committee make it even more urgent that we hear directly from the governor as soon as possible about this escalating scandal.”