There is a Morse Code message on the pause menu in each level as well as from messages from some of the dead MIBs (Men In Black). The following is a list of the decoded messages, which give you some insight into who the MIBs are and their goals:

York

1. Operation Deliverance: "Red forces encountered. Non-uniform opposition confirmed."
2. A Lone Survivor: No message
3. Spires: "Biological attack imminent. Stand down. Observe any resistance."
Grimsby

1. Fates Worse Than Death: "Red facility located. Study of infection path proceeding."
2. Conversion: "Friendly interception anticipated. Abort Grayskin capture."
3. Hunted Down: "Friendly intelligence chatter positive to Grayskin's resistance."
Manchester

1. Path Of Least Resistance: "Red anti air potential excessive. Hold off insertion."
2. The Cathedral: "Grayskin encounter with angel inevitable."
3. Outgunned: "Red tunnel network supports grid theory."
Nottingham

1. Into The Fire: "Friendly excavation investigation pending."
2. Conduits: "Red forces at friendly base in zone L770."
3. Viper's Nest: "Grayskin cut off in friendly base. Red forces present."