Trump's anti-mask + "Stop the Steal" madness = identifying rioters.

Children, siblings and spouses were horrified watching the insurrection — especially when they spotted people they knew in the crowd.

As millions of Americans watched scenes of pro-Trump mobs attacking the U.S. Capitol earlier this month, some viewers saw something familiar on their TV screens — their own family and friends.

The FBI has been leaning on spouses, siblings, children and former romantic partners who spotted their loved ones assaulting the Capitol and responded by dropping a dime on them.

"There are plenty of people, I don’t think it was so hard to report someone (even family)," said Michele Galietta, a psychology professor at John Jay College of Criminal Justice in New York City.

"Does that mean you enjoy it? No. I think it’s regrettable and I think most people would be upset they had to do it, but felt it was absolutely the right thing to do.”

Retired U.S. Air Force Lt. Col. Larry Brock was arrested after his former wife of 18 years contacted the FBI National Threat Operations Center on Jan. 8 saying she recognized him in footage.

"I just know that when I saw this was happening I was afraid he would be there," she told an FBI agent, according to an affidavit supporting charges against Brock. "I think you already know he was there. It is such a good picture of him and I recognize his patch."

Not long after the riot, insurrectionists knew they could be in trouble, with their largely unmasked faces in clear view, especially to family members and friends thousands of miles away.

Accused rioter Guy Reffitt, an apparent member of a Texas militia group, even threatened his adult children if they "crossed the line and reported" dad to authorities, according a criminal complaint.

Reffitt allegedly said he would "do what he had to do" to his own family members in such an event, prompting the young man to ask: "Are you threatening us?"

"(Reffitt) responded with words to the effect of, 'Don't put words in my mouth,'" FBI Special Agent Thomas Ryan wrote in the criminal complaint against Reffitt. "(The son) understood (Reffitt's) statements to be a threat to (the son's) life."

Little did Reffitt know, as he allegedly made that threat, son Jackson Reffitt had apparently already made that call.

"I got in contact with the FBI after the Capitol riots about my dad," the younger Reffitt told KDFW.

"It's this mob mentality and cult-like feeling to it, so there's a real concern of where we're headed and seeing a loved one being part of it," said Elizabeth Jeglic, another psychology professor at John Jay College.

"This is not Uncle Bob or Aunt Jane anymore. This is a mob of people attacking our nation, our freedom our values and that is not what your family values are about. When you see that, it can be very devastating."

[.nbcnews.com]