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's stake comes amid a time of struggle at the drugmaker, which has begun to cut costs aggressively as demand for its Covid-19 medicines recedes.

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