Apollo 13 Separation of the Lunar Module from the Command Module

Apollo13

Before the Command module from Apollo 13 began re-entry into the earth's atmosphere, the command & service module jettisoned the Lunar Module (LM). This is after Apollo 13 suffered major set backs in the mission. NASA and UTIAS came up with a solution in this separation.

I believe they either used pyrotechnic seperation (explosive bolts) or the expansion of gas in an area between the two modules. Any thoughts on how this separation took place?