Sirius and the Dark Mark

[hollygolightly95]

No: HPFGUIDX 35548

Greetings from London, England.
Apologies if this has been discussed before (couldn't find it in the 
archives...)

If all Death Eaters have the Dark Mark on their arm, it should be 
easy enough to identify them. If Sirius never was a Death Eater, he 
doesn't carry the mark, so why was that not enough to prove his 
innocence? Why was he sent to Azkaban without even a trial?

holly