[Spoilers] Explain a certain part of the ending in The Departed.

I just watched The Departed for the third time in my life just now and there is a part of the ending to that movie I did not understand and most likely missed something that happened earlier.

Leo has the gun at Matt at a rooftop and mentions that Martin Sheen's character had the wire tap recordings of Jack and Matt's conversation. Martin had the taps this whole time and as Leo also mentions that Martin had a contingency plan that if anything ever happens to him, his lawyer would give those tapes to Leo as evidence.

My question is why didn't Martin let the tapes lose already prior to his death?

There was a brief meeting of the 3 (Leo, Mark, and Martin) under the bridge and Leo asked Martin that he already had the evidence to bring Jack to justice but Martin needed more for it to 'stick'. I'm assuming he is also referring to the mole in the agency (Matt) as the last piece of the puzzle to bring the whole thing down.