A security fence around the perimeter of the White House. (Saul Loeb/Agence France-Presse via Getty Images)

SEVENTEEN MINUTES. That is a shockingly long time for an intruder to roam undetected on the grounds of the White House while the president is inside the executive mansion. Fortunately the intruder on March 10 was carrying nothing more lethal than two cans of pepper spray. But how could such a breach of security have occurred? Secret Service officials owe an answer. And while they are at it, they also might explain why they initially tried to minimize the seriousness of the incident.

The security failure, in which a 26-year-old California man with a history of mental illness pierced the outer perimeter of the White House near the Treasury Department, renews questions about the Secret Service’s ability to protect the country’s leaders and facilities. A series of embarrassments, including a 2014 incident in which an intruder with a knife managed to get into the White House before being tackled by an off-duty agent in the East Room, brought the agency under scrutiny. There followed an overhaul of management that supposedly tightened protections. That makes this latest incident, along with the theft of an agent’s laptop containing sensitive information about Trump Tower, all the more troubling.

“I worry this is the worst one yet . . . it scares me,” said Jason Chaffetz (R-Utah), chairman of the House Committee on Oversight and Government Reform. Mr. Chaffetz, describing a surveillance video viewed by the committee Monday in a closed-door briefing, detailed how the intruder cleared a fence and ground barriers, lingered on the south portico of the White House, moved through the south garden and peered in several windows before being apprehended. Only after Mr. Chaffetz raised questions did the Secret Service release a timeline of events disclosing that agents failed to detect the intruder for 17 minutes. A spokesman for the agency declined to comment when we asked why the agency had not been more forthcoming, a lapse particularly pronounced in light of President Trump’s (clearly premature) compliment about them doing a “fantastic job” in this case.

The Secret Service urgently needs to get its house in order. The agency is without a permanent director after the retirement of Joseph P. Clancy. In choosing a replacement, Mr. Trump would do well to take to heart recommendations about the need for someone from outside the agency to bring a fresh eye to its operation.