How do I report marriage fraud to inmigration authorities

For the US

USCIS Home Page

If your not sure that it's fraud take a second to think, if they conduct a huge investigation and bother these people and it turns out they are legal residents they could sue you for damages.

Always be sure, secondly the US Immigration service, USCIS, is backed up with these kinds of reports, they may never actually investigate your claims.

Yes, I gave you where to report, but don't expect them to actually investigate, heck, there are 50 or so illegals standing outside most home depos every week of the year, if they really wanted to do something with illegals.

Also if they were legally married, it is hard to prove fraud, just because they may separate or divorce, 1/2 of all marriage end in divorce anyway.