The Senate's main investigation into allegations of Russian meddling in the 2016 U.S. presidential election is equipped with a much smaller staff than previous high-profile intelligence and scandal probes in Congress, which could potentially affect its progress, according to sources and a Reuters review of public records.

With only seven staff members initially assigned to the Senate Intelligence Committee's three-month-old investigation, progress has been sluggish and minimal, said two sources with direct knowledge of the matter, who requested anonymity.

A committee aide, also speaking on condition of anonymity, said two more staff members were being added and a few others were involved less formally.