By Dustin Volz

WASHINGTON (Reuters) - The U.S. Senate on Wednesday voted down a Republican-backed proposal to expand the Federal Bureau of Investigation's secretive surveillance powers after the mass shooting at an Orlando gay nightclub last week.

The measure followed the Senate's rejection on Monday of four measures that would have restricted gun sales.

During Wednesday's vote, Senate Republican leader Mitch McConnell switched his vote to 'no,' giving himself the opportunity to bring the measure up for consideration again as soon as later this week.

The legislation would broaden the type of telephone and internet records the FBI could request from companies such as the Google unit of Alphabet Inc and Verizon Communications Inc without a warrant. Opponents, including some major technology companies, have said it would threaten civil liberties and do little to improve national security.

The legislation before the Senate on Wednesday, filed as an amendment to a criminal justice funding bill, would widen the FBI’s authority to use so-called National Security Letters, which do not require a warrant and whose very existence is usually a secret.

Such letters can compel a company to hand over a user's phone billing records. Under the Senate's change, the FBI would be able to demand electronic communications transaction records such as time stamps of emails and the emails' senders and recipients, in addition to some information about websites a person visits and social media log-in data.

It would not enable the FBI to use national security letters to obtain the actual content of electronic communications.

The legislation would also make permanent a provision of the USA Patriot Act that lets the intelligence community conduct surveillance on “lone wolf” suspects who do not have confirmed ties to a foreign terrorist group. That provision, which the Justice Department said last year had never been used, expires in December 2019.

The bill had been expected to narrowly pass but it fell two votes short of the required 60.

The future of the Senate proposal in the House of Representatives was also uncertain, given its alliance between libertarian-leaning Republicans and tech-friendly Democrats that has blocked past efforts to expand surveillance.

Privacy groups and civil liberties advocates accused Republicans this week of exploiting the Orlando shooting to build support for unrelated legislation.

Senator Ron Wyden, an Oregon Democrat, criticized Senate Republicans for “pushing fake, knee-jerk solutions that will do nothing to prevent mass shootings or terrorist attacks.”

Though Republicans invoked the Orlando shooting in support of the bill, FBI Director James Comey has said Omar Mateen’s transactional records were fully reviewed by authorities who investigated him twice for possible extremist ties.

Comey said there was “no indication” Mateen belonged to any extremist group and that it was unlikely authorities could have done anything differently to prevent the attack.

(Reporting by Dustin Volz; Editing by Steve Orlofsky and Bernard Orr)