The other states participating in the roughly 15-member group could not be learned. The sources asked not to be named because the investigation is not public.
The presence of a large number of attorneys general joining a Justice Department probe underscores the hurdles that both proposed combinations face in winning U.S. regulatory clearance.
Democratic presidential candidate Hillary Clinton, several lawmakers and the American Medical Association, a leading physicians group, have said they feared the pending acquisitions would hurt consumers by leading to higher insurance premiums or limited access to healthcare providers.