Pfizer Inc. (PFE) and Merck & Co. (MRK) are being pulled into an expanding congressional investigation about the agreement drugmakers reached with the Obama administration to support the Democrats’ overhaul of the U.S. healthcare system. The probe began last year with Republicans on the House Energy and Commerce Committee seeking documents from an industry trade group. When that group didn’t cooperate, the panel decided to target Pfizer, the world’s biggest drugmaker, along with Merck, Amgen Inc. (AMGN), Abbott Laboratories (ABT) and AstraZeneca Plc (AZN), said one of the people according to a recent report on Bloomberg.com.