Many have asked how would-be assassin Thomas Matthew Crooks was able to open fire on former President Donald Trump during a rally in Butler, Pennsylvania this past weekend.

Those questions grew more intense on Tuesday following a report that the sniper was previously spotted with a range finding tool. 

Officer took a picture of Crooks before shooting

According to the Beaver Countian, that sighting was made by a Beaver County Emergency Services Unit (ESU) counter-sniper who was stationed along with two others inside the building from whose roof the shooter fired.

"There were three counter-snipers located in the building that the shooter eventually used to take shots at Trump," one unnamed officer was quoted as telling the outlet.

One of the counter-snipers is said to have observed Crooks approach the building and look at its roof, which led the law enforcement officer to take a picture of him.

The counter-sniper later saw Crooks take out a laser range finder, which is a device used by rifle shooters to measure distance.

He responded by radioing his command post, after which Crooks disappeared but returned a short time later carrying a backpack.

Secret Service director: "Sloped roof" prevented agents from being deployed

Critics have demanded to know why security personnel were not stationed on the roof of the building that Crooks ultimately fired his weapon from.

Secret Service Director Kimberly Cheatle suggested in an interview with ABC News on Tuesday that the building's "sloped roof" prevented agents from being stationed on it.

"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," Fox News quoted Cheatle as saying.

Her argument was rejected by decorated combat veteran Sean Parnell who pointed out in a social media post that American "snipers used to set in on mountain tops in Afghanistan. On the down slopes if need be."

"The stupidity of this statement explains so much of why s--t hit the fan that day. Absolute incompetence," Parnell went on to complain.

Donald Trump Jr. calls roof claim "nonsense"

Donald Trump Jr. offered a similar perspective when speaking with Fox News host Sean Hannity at the Republican National Convention (RNC) on Tuesday.

"I can assure you … these are, especially the guys that I've known and they’re some of them will be lifelong friends, they’re not worried about a five-degree pitch on a sloped roof," The Hill quoted Trump as saying.

"Like, that's nonsense," the former president's eldest son said to Hannity. "That's not an excuse. It's the dumbest excuse."