Director of National Intelligence, General James Clapper says the reason why Russia allegedly meddled in the US election will be revealed in a report to be published next week.
Intelligence agencies say the Kremlin ordered the hack of Democratic emails.
Russia has denied involvement but the US has announced sanctions against Russian officials.
A report on foreign meddling in US elections was given to President Barack Obama on Thursday.
President-elect Trump is to be briefed on the report on Friday and the unclassified version will be made public next week.