Greg Vecchi, former chief of the FBI’s Behavioral Science Unit, said the agency is almost certainly preparing a worst-case scenario to take the occupied wildlife refuge headquarters by force, and that – following the procedures he was familiar with – had likely constructed a set of buildings that resemble the ones in Oregon and have practiced for a coordinated tactical assault.

“They have found a building or quickly put together a mock-up of the building and they’re practicing, putting role players in there to represent the people inside,” said Vecchi, who retired in 2014 and wrote his doctoral dissertation on hostage situations.

“They’re dropping helicopters, working flashbangs, coming in with [armored personnel carriers], so that if it happens, it’s kind of like the Navy SEALs did with Osama bin Laden,” he said. “Practice, practice, practice.”

But a tactical assault is an option of last resort and federal agents in recent years have taken a seemingly placid, wait ‘em out approach to potential showdowns like the one in Oregon. The more measured tactical plan was shaped by the pushback to the deadly sieges at Ruby Ridge, Idaho, and near Waco, Texas.