A member of a security group protecting rallygoers Sunday in downtown Portland helped federal officers arrest a masked demonstrator, prompting concerns from observers and a U.S. Attorney's Office review.

The incident has come under review because of the involvement of the security member, who is shown on video helping police detain a demonstrator near a rally of political conservatives at Terry Schrunk Plaza.

Video footage, including a clip taken by The Oregonian/OregonLive, shows a man who was providing security for free-speech advocates and backers of President Donald Trump retrieving a zip tie from an officer and helping subdue the demonstrator.

"The Federal Protective Officers were in the process of removing an unauthorized protester from the demonstration," U.S. Department of Homeland Security spokeswoman Lucy Martinez said in an email. "FPS officers were operating within the scope of their authority to ensure the safety of all persons and support the peaceful expression of their 1st Amendment rights."

The Portland Mercury first reported about the review.

A member of the public helping with an arrest isn't unusual, according to Portland's police chief.

"It's not uncommon if an officer is struggling with somebody or trying to arrest somebody, for a member of the public to come up and either render assistance, or just ask. So that's not that uncommon," Chief Mike Marshman told Oregon Public Broadcasting.

Mayor Ted Wheeler weighed in on the incident when confronted by protesters at a Wednesday council meeting, according to the Portland Tribune.

"I support the U.S. Attorney's investigation of that incident and that was not a Portland Police officer," Wheeler said, according to the newspaper. "That was a protective services officer who reports to Homeland Security, and as you'll recall, they're the ones who issued the permit for Schrunk Plaza. That was not a Portland Police Bureau action and I do not support it."

— Jim Ryan

jryan@oregonian.com

503-221-8005; @Jimryan015

Maxine Bernstein of The Oregonian/OregonLive staff contributed to this report