In January 2011, James Tomsheck, then a top internal affairs investigator inside US Customs and Border Protection, attended a meeting of about 100 senior CBP leaders in a hotel in Irvington, Virginia.

Amid the sanitized splendor of the hotel ballroom, he vividly recalls hearing the nation’s then highest-ranking border patrol agent, David Aguilar, laying out his vision for the future. Border patrol, the former CBP deputy commissioner said, was to become the “marine corps of the US federal law enforcement community”.

Another leading CBP figure remarked that border agents were not required to adhere to the same constitutional restraints on the use of force as other law enforcers. “We are not cops,” he said.

Fast forward to this month, when Tomsheck absorbed with mounting foreboding the images of federal officers – led by border patrol agents – wielding teargas and flash bangs against protesters in Portland, Oregon.

As news circulated of demonstrators being shot in the face with “less lethal” munitions, and of unidentified masked agents in camouflage strong-arming civilians into unmarked vans, the nightmare scenario Tomsheck had heard expressed by his bosses almost a decade ago – of border patrol becoming a nationwide militarized force operating outside constitutional constraints – was becoming real.

“Border patrol has always seen itself as a militarized force, and that aspiration is now being enabled by the current administration,” Tomsheck told the Guardian.

On Thursday, Trump ramped up his threat to send border patrol agents into US cities to tackle what he claims is an epidemic of violence and anarchy sweeping urban areas. He told Fox News he was prepared to send in 75,000 federal officers, warning: “We’ll go into all of the cities, any of the cities. We’re ready.”