[Cryptography] how to put a password in an evidence bag?

[ianG]

Many here will likely decline to follow the Bridges / Force arrests in 
USA, but in short, the two federal agents who were chasing the "Silk 
Road" website for bad stuff were also raiding the pot.

http://techcrunch.com/2015/04/01/feds-nabbed-for-big-bitcoin-heist-involving-gox-and-silk-road/#BhiiWz:b5eO

At one point, they arrested an employee of the website and got the admin 
password from him.  They then proceeded to raid the value that was 
escrowed within (customer funds) and sell it through an exchange 
provider - Mt.Gox.  Once they had extracted all the cash out via this 
exchange provider they proceeded to shut it down for running an 
unlicensed money transmitter ... you can't make this stuff up.

But the crux of this particular theft was getting the password to the 
accounts on silk road.  Now, passwords are "obviously" evidence that has 
been seized.  But how do you protect the chain of custody?  You can't 
exactly put it in an evidence bag ... or you can but that is not 
protecting it if one of the agents has copied it.

Does anyone know how the cops secure the password?  Or have they not yet 
begun to deal at this level.

iang