The Obama administration warned corporate America on Monday that the government will more aggressively investigate big firms that hurt smaller competitors - contending lax enforcement by the Bush administration fueled the current economic troubles. Assistant Attorney General Christine Varney said the Justice Department was abandoning legal guidelines established by the Bush administration in September 2008. Critics complained that the earlier instructions made it difficult to pursue antitrust cases against big firms. Varney laid out the new policy in a speech to the Center for American Progress, a left-leaning think tank. She said some of the economy's problems were due to the lack of enforcement in the previous 10 years - a clear jab at the Bush administration, which, she said, raised too many hurdles to antitrust investigations. "There was a high cost to standing aside. We must change course and take a new tack," said Varney. The new rules mark a return to the antitrust policies of the Clinton administration, which brought a major action against Microsoft. These days, similar questions are being asked about the market dominance of Google. The Justice Department is reviewing a proposed legal settlement with authors and publishers that would expand Google's digital library of books, after some librarians and consumer activists complained that the proposed settlement will give Google a digital monopoly on millions of books. Asked about Google, Varney insisted her remarks weren't aimed at any particular company or industry, but wanted all companies to get the message. "Look, when you become successful and you have market power, however you define it, you need to pay attention to the rules," she said. Bruce McDonald, a lawyer and former antitrust official in the Bush Justice Department, said Varney's comments did not give any clear indication whether the government will challenge conduct by Google. "We now know what direction Ms. Varney wants to take the antitrust enforcement, but exactly how far she will go will only be known when it's played out in particular cases," he said.