Infinite Crisis
It was revealed in the eighty-page one shot Countdown to Infinite Crisis (2005) that Maxwell Lord was in fact Checkmate's latest Black King, and had been collecting information on the Justice League members' weaknesses so that he could annihilate them. Blue Beetle had broken into Lord's secret headquarters and discovered his secret, but was murdered by Lord before he could warn anyone.
In The OMAC Project #5, the remnants of the Super Buddies (Mary Marvel included) are shown trying to avenge the Blue Beetle's murder, with no success.
Read more about this topic: Super Buddies