As someone who uncovered this finding and obtained valid evidence, then you alone have the right to tag it for the first time even though you are not currently a member of DT. This is the reason why you don't trust these users because of the cheating they did with alt on the same campaign. After that, you can ask DT for help to tag them.

So, I can advise you never to hesitate to tag the account you find cheating on the campaign with alt if the evidence you find is quite valid. I also did the same when I found them cheating on the same campaign, so I think this is the right choice if you join the club.