The US Secret Service did not put agents on the rooftop where an assassin shot at Donald Trump for health and safety reasons, the head of the agency has said.

Kimberly Cheatle, the Secret Service director, said the “sloped roof” where Thomas Matthew Crooks was positioned on Saturday could have posed a risk to agents.

“That building in particular has a sloped roof at its highest point. And so, you know, there’s a safety factor that would be considered there that we wouldn’t want to put somebody up on a sloped roof,” she told ABC News on Tuesday.

“And so, you know, the decision was made to secure the building, from inside.”

According to a Secret Service official, local police were stationed inside the Agr International building and radioed to alert agents to a man acting furtively, the Washington Post reported.

Joe Biden, the US president, has ordered a review into how Crooks was able to gain access to a point with a direct line of sight to Trump as he spoke at a rally in Pennsylvania.

A bullet fired at Trump grazed his ear, narrowly missing his head.