As President Donald Trump’s supporters massed outside the Capitol last week and sang the national anthem, a line of men wearing olive-drab helmets and body armor trudged purposefully up the marble stairs in a single-file line, each man holding the jacket collar of the one ahead.

The formation, known as “Ranger File,” is standard operating procedure for a combat team that is “stacking up” to breach a building — instantly recognizable to any U.S. soldier or Marine who served in Iraq and Afghanistan. It was a chilling sign that many at the vanguard of the mob that stormed the seat of American democracy either had military training or were trained by those who did.

An Associated Press review of public records, social media posts and videos shows at least 21 current or former members of the U.S. military or law enforcement have been identified as being at or near the Capitol riot, with more than a dozen others under investigation but not yet named. In many cases, those who stormed the Capitol appeared to employ tactics, body armor and technology such as two-way radio headsets that were similar to those of the very police they were confronting.