He stated this in Warsaw on Thursday, an Ukrinform correspondent reports.
"I think it was Russia and I think it could have been other people and other countries. It could have been a lot of people that interfered," Trump said.
He also accused former U.S. President Barack Obama, who, according to Trump, was aware of this situation, but did nothing to prevent it.
"When he was president, he found out about this in terms of if it were Russia, found out about it in August. Now the election was in November. That’s a lot of time. He did nothing about it," Trump said.