First, the lawyer most likely found the malware via an anti-virus software and did not detect a new malware specifically targeted to him. Second, the police probably used the drive at an unsafe machine.
Is it only me who finds this illogical nonsense? It sounds like if I get a "real world" infection like xyz flu - I have the right to seek criminal action against the person who got me that infection.
Even if you give the police the benefit of the doubt and assume zero malicious intent, the fact that malware appeared on the drive suggests either:
(A) Mishandling of evidence by cycling it through an insecure/unofficial system.
(B) Official systems have been pwned, and they are no longer trustworthy!
And the malware was inside a folder that had been created on the drive called "Bates Court Order". Meaning that the drive wasn't infected already, but somehow -after- the folder copying all the documents for discovery onto there, these several different pieces of malware all decided the best place to load themselves onto this drive was in this particular folder.