The head of the United States Secret Service, Kimberly Cheatle, is stepping down as Director of the Service over the assassination attempt against former President Donald Trump, after days of mounting pressure and criticism for failing to prevent the shooting.

Cheatle sent an email to staff on Tuesday announcing her plans to step down, The Associated Press news agency reported.

She had been under pressure from US lawmakers to resign as questions continue to swirl around security protocols at Trump’s July 13 rally in Pennsylvania, where the former president and Republican presidential nominee was shot in the ear.

“I take full responsibility for the security lapse,” Cheatle said in the email to staff. “In light of recent events, it is with a heavy heart that I have made the difficult decision to step down as your director.”

US President Joe Biden thanked the Secret Service director for her decades of public service and said he planned to appoint her replacement soon.

“The independent review to get to the bottom of what happened on July 13 continues, and I look forward to assessing its conclusions. We all know what happened that day can never happen again,” Biden said in a statement.

US Homeland Security Secretary Alejandro Mayorkas also said later in the day that he had appointed Ronald L Rowe, the Secret Service’s deputy director, to serve as acting director following’s Cheatle’s resignation.

Cheatle had faced angry questions during a US congressional committee hearing on Monday over the agency’s security plan at the Trump rally.

The ex-president was shot in the ear by a gunman that witnesses say had taken up a position on a rooftop with a direct line of sight of the rally stage, heaping pressure on the Secret Service to explain what measures were put in place before the event.

“The assassination attempt on former President Donald Trump on July 13 is the most significant operational failure at the Secret Service in decades,” Cheatle told lawmakers during the House Oversight and Accountability Committee hearing.

“There clearly was a mistake and we will make every effort to make sure that this never happens again,” she said.

Biden has ordered an independent review of security protocols at the rally, where one attendee was killed and two others were seriously injured.

But lawmakers had been pushing for more immediate answers from Cheatle and other US law enforcement leaders, including Mayorkas in the Department of Homeland Security and FBI Director Christopher Wray.

Republican House Speaker Mike Johnson told reporters on Tuesday morning that Cheatle’s resignation was “overdue”.

“Now we have to pick up the pieces; we have to rebuild the American peoples’ trust and trust in the Secret Service as an agency. It has an incredibly important responsibility in protecting presidents, former presidents and other officials in the executive branch,” Johnson said.

“And we’ve got a lot of work to do.”

Reporting from Washington, DC, Al Jazeera’s Mike Hanna noted that Cheatle’s decision to step down comes at a critical moment, just months before a US election in November.

Trump is now expected to face off against the Democratic Party’s presumptive nominee, Vice President Kamala Harris, after Biden withdrew from the race on Sunday.

“The director of the Secret Service is an incredibly important function,” Hanna said.

“It’s now up to the president to appoint a new director. Obviously [it will be] a very critical appointment at a time when the country is going into a national election, at a time when there is immense political divide.”