Alice and Bob

Just wanted to have a nice private conversation... Until the police stepped in.
http://www.websequencediagrams.com is cool
Here's my source:
note over Alice,Bob: Want to have a secure conversation
note over Bob,Frank: Roommates
Police->Alice: Taps Alice's line
Bob->Police: Pays off
activate Bob
Police->Bob: Informant
deactivate Bob
Bob->Alice: Stabs
note over Alice: Dies
Alice->Bob: Falls Down onto
Bob->Frank: Asks to borrow shovel
Frank->Bob: Lends shovel
Bob->Alice: Buries Body
Alice->Alice: Decomposes
Frank->Police: Calls the Authorities
Police->Bob: Arrest