The Secret Service is once again facing heat after the agency's acting director Ronald Rowe Jr. admitted the West Palm Beach golf course wasn't searched prior to the 2nd assassination attempt on Donald Trump's life.

Rowe -- who stepped into the role after Kimberly Cheatle resigned following the July shooting at Trump's Pennsylvania rally -- spoke with reporters Monday, and confessed an extensive search of the grounds was not conducted during 45's visit to the Trump International Golf Club in Florida Sunday ... defending the visit was "not on [Trump's] official schedule."

He added ... "The president wasn’t even really supposed to go there ... and so we put together a security plan -- and that security plan worked."

The Secret Service has been under an immense amount of scrutiny this summer ... after Thomas Matthew Crooks fired at the former president from a rooftop at the Republican nominee's rally in Butler, Pennsylvania on July 13.

The first assassination attempt has been described as the agency's "worst'' failure in decades ... so, it isn't surprising many are clamoring for answers following the 2nd would-be assassin's attempt.

Rowe didn't clarify whether this "off-the-record" plan meant the agents had enough time to sweep the Florida golf course or not ... though, he did praise the agents on the scene for spotting the suspected gunman, who has since been identified as Ryan Wesley Routh.

As TMZ previously reported ... Secret Service agents clocked an AK-style rifle peeking out from some shrubbery ... about 400 yards away from where the former prez was teeing off.

The suspect -- who never fired his weapon -- allegedly dropped the rifle and fled in a nearby SUV after an agent spotted him and opened fire into the area. Authorities said they later discovered at the site a loaded "SKS-style" rifle with a scope, 2 bags -- one with food in it -- and a digital camera. Investigators believe the suspect had been lying in wait for about 12 hours.

Per authorities, the rifle in question had a serial number that was "obliterated and unreadable."

Routh was later stopped and arrested by the Martin County Sheriff's Department ... who performed the traffic stop on a Florida highway after getting an alert from the Palm Beach County Sheriff's Department.