Hillary Clinton on Wednesday said she believes that Russians likely received help from inside the U.S. on how to effectively use the information that intelligence agencies say was gathered to meddle in last year’s presidential election, which she lost to President Donald Trump.

“The Russians, in my opinion and based on the intel and counterintel people I’ve talked to, could not have known how best to weaponize that information unless they had been guided,” said Mrs. Clinton at the Code technology conference in Rancho Palos...