Sens. Elizabeth Warren, D-Mass., and Richard Blumenthal, D-Conn., initiated an investigation into the National Labor Relations Board on Wednesday concerning the agency's decision to drop charges against Elon Musk's SpaceX over retaliatory firings, according to correspondence first obtained by CNBC. SpaceX is reportedly seeking to go public in June and eyeing a $2 trillion valuation . Under the Biden administration, the NLRB had charged the aerospace and defense firm with illegally firing