9:21 AM PT -- Director Kimberly Cheatle just acknowledged that the Secret Service did not pause the rally despite the fact that agents and others saw Thomas Matthew Crooks on the roof and around the fairgrounds well before Donald Trump was up onstage talking.

She didn't have a clear answer for why officials didn't get Trump outta there despite this ... simply saying they're still looking into who communicated what to who, and when.

The Secret Service Director is getting grilled on Capitol Hill over the assassination attempt on Donald Trump -- and she's acknowledging the Secret Service screwed the pooch.

Kimberly Cheatle is in the middle of testifying before the House Oversight Committee -- and right from the outset, she took full responsibility for the near-death of the ex-Prez ... saying the agency failed in its duty to protect DT.

Specifically, she called the incident "the most significant operational failure" at the Secret Service in decades.

Amid calls for her resignation, Cheatle vowed she'd move heaven and earth to ensure an incident like this never happened again. Despite previously pointing the finger at local law enforcement in post-shooting interviews ... Cheatle backtracked here in her testimony, voicing support for officials on the local and federal levels.

KC also said changes would be coming to the agency well before an investigation concludes about the assassination attempt.

As U.S. representatives grilled Cheatle with questions, she danced around explaining why the Secret Service did not have someone stationed on the rooftop at the rally.

However, she did confirm the shooter, Thomas Matthew Crooks, was identified as a suspicious person before the shooting ... but noted he hadn't been labeled a "threat." Cheatle also confirmed Crooks had a rangefinder on him at the rally ... but could not share if he was ever questioned about the item.

Per KC, a rangefinder is not a prohibited item at an outdoor event ... despite its frequent use as a hunting tool.

Cheatle first spoke on the attack during a sit down with ABC News correspondent Pierre Thomas ... where she called the whole ordeal "unacceptable."

She said the attempted assassination was ultimately on her -- as she made it clear the shooter should've never been allowed to make it onto the rooftop at Trump's rally. She confirmed a review would be coming in the aftermath of the assassination attempt -- which left Trump shot in the ear and another dead.

As we reported ... Crooks was shot and killed by Secret Service agents after he opened fire on the crowd in Butler, Pennsylvania with an AR–15–style rifle.

Though Cheatle defended the whole scene played out over a short period of time, multiple reports claim he was spotted on the rooftop 20 minutes before he shot his first bullet.

Congress members have a lot of questions for her -- we'll see what she has to say.