The building from which a gunman fired at former President Donald Trump on Saturday was — at least in hindsight — an obvious security risk. Its rooftop offered an ideal sniper’s perch, with a close, elevated and unobstructed view of Trump.

But when the Secret Service drew up plans for Saturday’s rally, it left that building outside its security perimeter. Instead, local law enforcement officials in Butler, Pennsylvania, were given responsibility for that building, and no police officers were stationed on the roof itself.

The building, used as a warehouse by equipment manufacturer AGR International, has become a focal point of myriad investigations into the shooting that nearly felled a former US President, one that homeland security secretary Alejandro Mayorkas on Monday called a security failure.

The first question is why the building, about 137 metres from the stage, was left out of the perimeter. A Secret Service advance team visited the site and made the determination, and a supervisor would have had to approve it. The agency so far has not said who that was.

That is just one of many unanswered questions. It is also unclear how the gunman got on the roof. People at the rally reported a suspicious person to local law enforcement. Quickly thereafter, rally visitors pointed out a man on the warehouse and the Secret Service shot and killed him after he began his assault.

The Secret Service has not said whether local law enforcement officers made service agents aware of a suspect, or whether those officers were up to the task of dealing with the situation.

But the central question is whether the Secret Service failed at its most basic mission: keeping America’s leaders, including a former President, safe.

The security lapse is now the centre of overlapping investigations, run by members of Congress and the Secret Service. On Monday, the leaders of the Senate’s Homeland Security Committee announced their own investigation into Saturday night’s shooting. Senators Gary Peters and Rand Paul told the Secret Service in a letter that they wanted to know “how the suspect was able to get this close to a Secret Service protectee”.

“An incident like this cannot happen,” Mayorkas told CNN, adding, “When I say something like this cannot happen, we are speaking of a failure.”

Former federal law enforcement officials have said the Secret Service should have ensured the building was secured before the rally took place. The agency often relies on local law enforcement agencies for security at events.

It is still not clear when and how the gunman got on the roof of the warehouse. Anthony Guglielmi, a Secret Service spokesperson, said on Sunday that local law enforcement had been notified of a suspicious person by rally visitors before the event started.