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President-elect Donald Trump on Saturday again expressed skepticism about U.S. intelligence conclusions that Russia was involved in a covert campaign to interfere in the presidential election, and said "hacking is a very hard thing to prove."

Trump made the comments to reporters outside his Mar-a-Lago home in Florida, where the president-elect was holding a New Year’s Eve bash.

"I just want them to be sure, because it’s a pretty serious charge, and I want them to be sure," Trump said of assessments that Russia, and possibly its president Vladimir Putin, was involved in the campaign of cyber-attacks and attempted interference in the electoral process.

"And I know a lot about hacking. And hacking is a very hard thing to prove," Trump said. "So it could be somebody else. And I also know things that other people don't know, and so they cannot be sure of the situation."

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Trump did not elaborate, but when asked what things he knows he replied, "you'll find out on Tuesday or Wednesday."

The comments come two days after the U.S. announced sanctions and other retaliatory steps against Russia over the hacking of Democratic Party organizations and officials during the presidential campaign.

Related: Vermont Electricity Department Finds Malware Linked to Russian Hackers

Trump has said he does not believe Russia tried to interfere with the presidential election.

Russia has repeatedly denied any involvement.

Asked how important cyber-security would be in the Trump administration, the president-elect said extremely sensitive information should not be emailed or sent electronically.

"If you have something really important, write it out and have it delivered by courier, the old fashioned way because I'll tell you what, no computer is safe," Trump said.

"I have a boy who's ten years old; he can do anything with a computer. You want something to really go without detection, write it out and have it sent by courier," Trump said.

Trump said in a statement Thursday after the sanctions were announced that "it's time for our country to move on to bigger and better things" but that he would meet with U.S. intelligence officials next week to be briefed on the claims against Russia.

Asked about what questions he planned to ask, Trump didn’t get into specifics but added: "Hopefully we're going to have great relationships with many countries, it includes Russia and it includes China."