AFAIK, it is up to the DHS people to detain and ask questions. I've met other US vets returning from Ukraine that were not detained and questioned.
The American served 5 years in the US Marines, had a TS/Crypto clearance, multiple combat deployments and 3x PHs. He said it was the first time he know of a Ukraine volunteer was detained.
He said the DHS event in Denver was not a standard briefing with civil questions. He said it was more like an inquisition with a bright light in his face being shouted at and accused as a Nazi and sympathizer and wanting names and unit designations. It probable didn't help having a very Germanic first name. He said he was very uncooperative which probably did not help. He said they didn't care if he misses his flight, so it appears it was on purpose.
I was in the one in Vancouver and it was very civil and only 45 minutes and we didn't miss our plane. I was surprised they scanned my passport into the system.
The Canadian authorities did not hassle us at all. I understand, the US government guys were just doing their job.
He has no Nazi tattoos or social media regarding any type of "extremism." As a reference they could have called General Budanov, Ukraine Head of Military Intel who he reported to. He didn't have any military manuals or equipment.
I spent 2 weeks traveling throughout Ukraine at the end of 2024 with him and met Western fighters from many countries. I didn't see any visuals or get any Nazi vibes. That includes traveling through Lvov, Kyiv, Kharkiv, Zaporezia, Odesa, and other towns.
The Azov units, who are mostly in SE Ukraine and Donbas, are the ones with the Nazi agenda. Most have been fighting since 2014 and are actually stand alone units that mostly support themselves financially, training and equipment. Their recruiting posters are in every city and town.
I met Westerners that fought along side with Azov and said they are one of the best fighting units with the highest morale. They did not have to swear a blood oath to the Fuhrer.
You don't need to be a Nazi follower and not every Ukrainian serving in Azov units is a Nazi but they do attract people with that viewpoint. In the last few years they have somewhat "cleaned up" their act playing down the Nazi part for better PR. I'm unsure of their status and reputation now.
So while we were being detained at the airport as potential Nazis and White Supremacist's, the same DHS was letting guys with "MS13" tattooed on their forehead with known criminal records (MS13 was labeled as a Transnational Criminal Organization) enter the country free and not tracked. Go figure.
Yes, it was nice to get FBI the letter but he was already aware of the sites he's on as a mercenary and Nazi.
He's now a consultant for the US Army and Marines and doing foreign military training based on his experience and drone ops in Ukraine and over one year in Bakhmut as an Intel Officer Team Leader. He also has his TS clearance back.
Correction: IIRC it was US Customs and Border Protection, not DHS.
Wolfhag