WASHINGTON (Reuters) - More than a quarter of the Senate's members asked the top intelligence official on Friday to release more information on the government's bulk collection of data on Americans' communications.

Led by Senator Ron Wyden, an Oregon Democrat who has long hinted at the broad scope of the classified surveillance, the group of 21 Democrats, four Republicans and one independent sent a letter to James Clapper, the director of National Intelligence, requesting public answers to a series of questions about how the data is collected and used.

Former National Security Agency contractor Edward Snowden faces espionage charges in the United States for taking records about secret surveillance of Internet and phone traffic and releasing them to the Guardian and The Washington Post.

Snowden, who has requested political asylum in Ecuador, has not been seen since he arrived in Moscow on Sunday. Russian officials said he was in a transit area at Sheremetyevo airport.

The release of the material has prompted a firestorm of concerns about the extent of government data tracking as well as questions about lawmakers' role in approving the legislation that allowed the data collection.

Critics of the surveillance programs see them as infringing on Americans' privacy rights, while backers said they are important tools for national security and subject to close control by the courts.

The senators said the purpose of their letter was to foster debate.

"Reliance on secret law to conduct domestic surveillance activities raises serious civil liberty concerns and all but removes the public from an informed national security and civil liberty debate," the senators said.

In the letter they said it was "regrettable" that information on the programs came through a leak to the media instead of from the Obama administration, "but we appreciate the comments that the president has made welcoming debate on this topic."

Among other things, the letter asked for answers about how long the National Security Agency has been engaged in bulk collection of communications records and whether there have been violations of court orders pertaining to the records.

It also sought examples in which useful intelligence was gained from the records and asked if there were plans to collect other forms of information under the Patriot Act, which was enacted after the September 11, 2001, attacks on the United States.

(Reporting by Patricia Zengerle; Editing by Vicki Allen)