The Justice Department on Monday gave more details about the review Attorney General William P. Barr has ordered into the government’s surveillance of President Trump’s 2016 campaign, saying there are still too many “open questions” about why it was done.

Assistant Attorney General Stephen E. Boyd avoided saying Mr. Trump was spied upon — forgoing the language Mr. Barr has used — but did say the “U.S. government and others undertook certain intelligence-gathering and investigative steps” toward Mr. Trump’s campaign.

“The purpose of the review is to more fully understand the efficacy and propriety of those steps,” Mr. Boyd said in a letter to House Judiciary Committee Chairman Jerrold Nadler.

John Durham, the U.S. attorney whom Mr. Barr tapped the lead the review, will do double-duty, continuing to serve as Connecticut’s top federal prosecutor while also working out of Washington to lead the review team.

Its funding will come from general money available to federal prosecutors at the department, Mr. Boyd said. He did not speculate on the size of the budget nor the length of the probe.

The Justice Department did defend Mr. Trump’s grant of declassification powers to Mr. Barr for purposes of letting the public see what the review generates. Mr. Boyd, in his letter, said the department has already issued a directive to intelligence community agencies to preserve their records and to make their people available to cooperate.

And Mr. Boyd said Mr. Barr will be judicious in his use of the declassification powers Mr. Trump granted him.