“I agree with Secretary Pompeo’s assessment: It certainly appears to be the Russians,” Mr. Barr said, further undercutting President Trump’s effort to cast doubt on whether the government of President Vladimir V. Putin of Russia was behind the attack. Mr. Mnuchin was among several top officials in the government who met with national security officials for the first time at the White House on Monday to assess the damage and discuss how to deal with it. It was held two days after Mr. Trump said the attack on federal networks was “under control,” was being exaggerated by the news media and might have been carried out by China rather than Russia, which has been identified by intelligence agencies, other government officials and cybersecurity firms as the almost certain source of the hacking. 