WASHINGTON — Federal immigration and health officials were blindsided by President Donald Trump’s “zero tolerance” policy on migrants crossing the Southwest border, triggering a cascade of problems as agencies struggled with the fallout from family separations, congressional investigators said in a report issued Wednesday.

The investigation by the nonpartisan Government Accountability Office dissects the consequences of launching a major policy change without consulting with the agencies that have to carry it out. The report makes no recommendations as it reconstructs a turbulent episode for the Trump administration.

“Officials told us that the agencies did not take specific planning steps because they did not have advance notice,” the GAO found. “Officials we interviewed stated that they became aware of the April 2018 (‘zero-tolerance’) memo when it was announced publicly.”

On April 6, Trump issued an executive order directing an end to “catch and release” at the border, a practice whereby migrants could be admitted into the country temporarily while their immigration claims were pending. That day, Attorney General Jeff Sessions ordered federal prosecutors to adopt a “zero tolerance policy” for border crossers. Parents placed into criminal custody could not keep their children with them, so families were split.

The report focuses on Homeland Security agencies such as Immigration and Customs Enforcement and the Border Patrol, part of U.S. Customs and Border Protection, along with the Health and Human Services Office of Refugee Resettlement, or ORR, which traditionally has had responsibility for providing shelter for unaccompanied minors detained at the border.

The HHS agency had a different task this time because the children were deemed “unaccompanied minors” after the government had separated them from their parents. Previously the refugee office was used to dealing with minors who arrived at the border alone. There was another difference: Many children who were separated from their parents were younger than the teens and pre-teens ORR normally attended to.

“Shelters converted space previously used for classrooms for older children to be space for children under age 5, with one shelter adding cribs, smaller tables and chairs, and toys appropriate for younger children,” the report noted.

The GAO found 103 children under the age of 4.

Under political pressure, Trump ultimately rescinded family separation. A federal court ordered the government to reunite families and provide regular progress reports. But the agencies still had a problem: Their computer systems often did not flag when a child was separated from his or her parents. As a result, it wasn’t always possible to verify that fact. ORR said that Homeland Security sometimes did not provide the information.

The GAO said administration officials as of Sept. 10 identified 2,654 children who were covered by the court order reuniting families. However, that number doesn’t include all the children affected, because not all families were covered by the court order. The report found 2,217 children who were part of the court case had been released from federal custody as of Sept. 10. About 90 percent of them reunited with a parent, although some were placed with sponsors.

About 250 children are still in custody, according to a report from the American Civil Liberties Union.