Pfizer appointed a former chief executive officer at Vanguard, the drug giant's biggest investor, to its board on Tuesday as it faces pressure from activist hedge fund Starboard Value.

Activist investor Starboard Value claims the pharma giant’s former CEO and CFO were “threatened” into backing current management.

The corporate clash between Pfizer and Starboard Value heated up on Thursday when the activist investment firm pressed the U.S. drugmaker to investigate actions by its board, after two former executives backed out of its campaign against the company.

Starboard has accused Pfizer of “coercive conduct” after executives who initially backed the hedge fund retracted their support.