- For Double Agents working from within the CIA, see Double Agent (CIA)
A Double Agent is a Participant in a Criminal Plot who has been convinced to work for the CIA. Max can turn an enemy agent into a Double Agent by collecting Incriminating Evidence about him from a Floor Safe 
in that agent's hideout.
Double Agents continue to perform their role in the plot - up to a point - and will not raise the suspicion of other agents nor disrupt the flow of the plot. They cannot commit Major Crimes, nor can they go into hiding. Turning an enemy into a Double Agent earns you twice as many points as simply arresting him.
Turning an Enemy Agent[]
Incriminating Evidence found.
To turn an enemy agent into a Double Agent (in short: "Turning" an agent), max needs to find Incriminating Evidence - information that points so clearly against a suspect that they would rather cooperate with the CIA than otherwise.
Incriminating Evidence is quite rare, and can only be collected from a specific place at a specific time, though it usually is worth it. It can only be found inside a Floor Safe , and you are not guaranteed that any will appear in the correct location to begin with.
Only Participants in a plot can be Turned. If any Red Herring Agents are found during a mission, none of them may be turned at all (you can't find incriminating evidence about a person not involved in anything illegal...).
Finally, it is possible to turn all agents involved in a plot - except the Mastermind - though this would be extremely rare.
Location[]
To find Incriminating Evidence about an agent, Max must search that agent's hideout. That means locating the hideout in the city the agent resides in, and Breaking into that hideout. You cannot find Incriminating Evidence about an agent in a hideout that does not belong to that agent.
The evidence itself can only be found in a Floor Safe.
Floor Safe[]
To collect the Incriminating Evidence, Max must crack a Floor Safe and photograph the documents inside. To do this, Max locate a Floor Safe at the agent's hideout.
Floor Safes may or may not exist at the agent's location. Due to the semi-randomal nature of building layouts, it is possible that no Floor Safes can be found there at all. Re-entering an agent's hideout will not re-randomize the layout, so if his hideout does not contain a floor safe, it never will.
To crack a Floor Safe, Max must be equipped with a Safe Cracking Kit 
. To photograph the documents inside, Max must be equipped with a Camera 
. You must do both to obtain the Incriminating Evidence.
Note that the kind of documents stashed in each Floor Safe is randomized right before that Floor Safe is photographed. There is no guarantee that photographing a Floor Safe will yield such evidence, even if all other conditions are met. Re-entering a hideout will allow collecting more info from each Floor Safe in that hideout, so a building with at least one Floor Safe may be re-entered over and over again, and that Floor Safe inspected each time, and will eventually yield the correct info - though this can take many tries. Remember that time is usually of the essence, so do not count on this being possible.
Solid Evidence[]
For Incrimnating Evidence to appear in Floor Safes , one condition must be met: You must already have knowledge of the agent's Role in the plot recorded in that Agent's Suspect File.
This knowledge can be gathered in several different ways, including interrogation of other agents, searching their name on a suitable Terminal, or collecting it from a Floor Safe. Until you obtain this knowledge, Incriminating Evidence will not appear in Floor Safes at all.
Also note that the chance of getting Incriminating Evidence from a floor safe is considerably lessened if you do have not already acquired a Master Plan list for the current plot, or a Personnel File for the agent's organization. Either of these will usually appear in Floor Safes instead of Incriminating Evidence until both are found.
The Arrest[]
Finally, to turn the agent once Incriminating Evidence was found, Max must Arrest that agent during the same break-in in which he found that Evidence! If the building is left without arresting the agent, the Incriminating Evidence is lost, and must be collected again if you still wish to turn that agent.
Note that as far as the Arrest itself goes, it is exactly the same as without the Incriminating Evidence, until the point where the Agent is brought to the CIA office. In other words, that agent will still raise the alarm if he sees you without a Disguise, and will raise the alarm when nabbed from his chair.
Effects[]
Once an enemy agent has been turned, his behavior as part of the Criminal Plot will change in several ways.
Going into Hiding[]
For one, that agent cannot go into hiding anymore. Even if the agent completes his mission, or has to perform an action involving another agent who is already In Hiding or Arrested, the Double Agent will not disappear.
Interrogation Info[]
In addition, Double Agents will always yield all information relevant to them during their interrogation. This includes all details and content of messages sent to other agents (as well as all details about those agents), as well as all details and content of meetings with other agents.
Future Message Traffic[]
Double Agents will also immediately inform you whenever they send a message to other agents or meet them in person. This will generate a Bulletin containing the contents of those messages/meetings, and will add the relevant information to the other agent's Suspect File.
Plot Participation[]
Finally, Double Agents will continue performing their role in the plot to the best of their ability. This includes sending messages, meeting other agents, and even performing Minor Crimes to obtain items or information.
However, Major Agents who are turned will not perform the Primary Crime of the plot, assuming they haven't done so already. This means that turning a Major Agent is an exceptionally favourable outcome - they will not cause the plot to break, but the Primary Crime is prevented from occuring nonetheless. Other agents will continue attempting to provide the Major Agent with Items as required, until they can go into hiding or are themselves Arrested or Turned.
Scoring[]
When an enemy agent is Turned, and kept Turned until the end of the mission, you will receive double the score for that agent as you would by simple Arresting them. However, since a Perfect Score requires you to Arrest all the participants, Turning an agent makes it impossible to get this unless you go back in and Arrest him/her.
Losing a Double Agent[]
It is possible through carelessness to lose a Double Agent. This occurs if that same agent is Arrested a second time without re-acquiring Incriminating Evidence about that agent just before arresting him.
If this is done, the agent is now considered Arrested, and counts as a regular Arrest for all intents and purposes. That agent no longer participates in the plot, meaning that the plot may break around him. In addition, your score for arresting that agent goes back to normal (instead of double).
This cannot be reversed: once an agent is Arrested, you cannot find them a second time to Turn them - they will stay Arrested until the mission is over.