Police officer working undercover as an inmate

Is there a term for a police officer or someone else working for a country's security forces who masquerades as an inmate and is put into a cell with a suspect in order to extract information?

I'm not sure that informant works here..

I think most people would use the informal and colloquial word mole:

4 : a spy (such as a double agent) who establishes a cover long before beginning espionage
broadly : one within an organization who passes on information

Wikipedia says more about its applicability to police officers specifically:

In police work, a mole is an undercover law-enforcement agent who joins an organization in order to collect incriminating evidence about its operations and so bring its members to justice.

I think I would probably use infiltrator as a general term, though more specifically, in this situation an undercover police officer.

As the below quotation from the OED will illustrate, the verb infiltrate is used figuratively, in a political sense, normally to describe a process of political subversion. However armies and law enforcement authorities can and have infiltrated the hierarchies of insurgent and subversive movements in order to gain information.

It has been a method, used by Britain in colonial situations against such as the Mau Mau in Kenya, and the Communists in Malaya. It was also a process which played a big part in the destabilisation of the IRA in Northern Ireland.

Police forces infiltrate criminal gangs, and hence the placing of an undercover officer in a cell would seem to me to amount to much the same thing - to gain information on a wider criminal organisation, or group of accomplices.

Infiltrate-verb 4. Military. transitive and intransitive. To penetrate (enemy lines) by the gradual or surreptitious movement of small numbers of troops; to move (one's own troops) surreptitiously into the enemy's lines. Also figurative, esp. for the purpose of political subversion.