WASHINGTON (Reuters) - Former U.S. spy agency contractor Edward Snowden might voluntarily return to the United States if given assurances of his constitutional rights, his father said in a letter to Attorney General Eric Holder.

Lonnie Snowden was "reasonably confident" his son, who faces espionage charges in the United States for alleged leaking of secret surveillance information, would return if certain conditions were met, the June 27 letter said. It was written by a lawyer on Snowden's behalf, and was obtained by Reuters.

The younger Snowden, a former contractor for the National Security Agency, should not be detained or imprisoned before trial, should not be subject to a gag order, and should be tried in a venue of his choosing, the letter said.

Edward Snowden, a U.S. citizen, fled the United States to Hong Kong in May, a few weeks before publication in the Guardian and the Washington Post of details he said he provided about U.S. government surveillance of Internet and phone traffic.

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

Representatives for the Justice Department could not be reached immediately for comment on the letter.

Lonnie Snowden said he was concerned that his son was being manipulated by others, including people from the anti-government secrecy group WikiLeaks, he said in an interview on NBC television earlier on Friday.

"I am concerned about those who surround him," he told NBC. "Wikileaks - if you look at past history - their focus isn't necessarily the Constitution of the United States. It's simply to release as much information as possible. So that alone is a concern for me."

Snowden's father said he has not had contact with his son since April, NBC reported.

"I love him. I would like to have the opportunity to communicate with him. I don't want to put him in peril," he said in the interview.

Snowden said he did not think his son had committed treason, even though he said Edward Snowden broke U.S. laws in releasing details about the federal monitoring programs.

"He has betrayed his government, but I don't believe that he's betrayed the people of the United States," he said.

(Additional reporting by Douwe Miedema; editing by Vicki Allen and Jackie Frank)