WASHINGTON (AP) — President Donald Trump appeared to cast doubt on the assessment of 17 U.S. intelligence agencies that blame Russia for election meddling, questioning Thursday why the Obama administration didn't try to stop it.

"By the way, if Russia was working so hard on the 2016 Election, it all took place during the Obama Admin.," the president tweeted. "Why didn't they stop them?"

All 17 intelligence agencies have agreed Russia was behind the hack of Democratic email systems and tried to influence the 2016 election to benefit Trump. The findings are at the heart of an investigation into contacts that members of Trump's campaign team may have had with Russian officials during the campaign and the transition.

Trump, frequently lashes out at the Russia investigation as a "witch hunt" spearheaded by Democrats.

He tweeted Thursday that the Democratic National Committee turned down an offer from the Department of Homeland Security "to protect against hacks (long prior to election). It's all a big Dem HOAX!"

"...Why did the DNC REFUSE to turn over its Server to the FBI, and still hasn't? It's all a big Dem scam and excuse for losing the election!" he wrote.

A day earlier, former Homeland Security Secretary Jeh Johnson told the House intelligence committee that in the late summer and into the fall, he was very concerned about the meddling in state election systems and that the department encouraged states to seek assistance from DHS. He said he was frustrated DHS learned of the hack into the DNC late in the game and that the committee refused help because it was using a private cyber security firm.

"In retrospect, it would be easy for me to say that I should have bought a sleeping bag and camped out in front of the DNC in late summer," Johnson said.

Johnson also addressed the Obama administration's political sensitivity when it came to warning of the Russian meddling, and alluded to problems created at the time by Trump's own statements.

"One of the candidates, as you'll recall, was predicting that the election was going to be rigged in some way. And so we were concerned that, by making the statement, we might in and of itself be challenging the integrity of the — of the election process itself," Johnson said.

Last month, Trump fired FBI Director James Comey, who was leading the agency's Russia probe. The president has come under harsh criticism by some who claim he threatened to undermine the investigation by firing Comey.

Special counsel Robert Mueller was later named to lead the investigation, and The Washington Post reported that Mueller is considering investigating Trump for obstruction of justice because he fired Comey.

The investigation has shadowed Trump from the outset, though he's denied any ties to Russia or knowledge of any campaign coordination with Moscow.

Trump also claimed Thursday that Johnson "is latest top intelligence official to state there was no grand scheme between Trump & Russia." But Johnson didn't say that Wednesday. He said he wasn't aware of efforts by Trump or his campaign to collude with Russia beyond what the intelligence community already knows. Johnson also said Russian hacking didn't change election totals, but added that he can't be sure other meddling didn't influence public opinion.

"It is not for me to know to what extent the Russian hacks influenced public opinion and thereby influence the outcome of the election," he said.

Trump has picked fights with intelligence agencies in the past, blaming them for leaks about his associates' Russia ties. During the transition before his inauguration, he ripped into the intelligence community for being behind the leaks and even compared them to Nazi propaganda. "Intelligence agencies should never have allowed this fake news to "leak" into the public. One last shot at me. Are we living in Nazi Germany?" he tweeted in January.

White House spokeswoman Sarah Huckabee Sanders said Thursday that while the president doesn't think the election results were influenced by Russia, he has "made it clear that we have to protect the integrity of the electoral process."

Sanders pointed to comments Trump made at a January news conference, underscoring that he has not dismissed the idea that Russia hacked the U.S. election, but he also believes "we also get hacked by other countries and other people."