The National Security Agency has been campaigning to lead the government's rapidly growing cybersecurity programs, raising privacy and civil liberties concerns among some officials who fear that the move could give the spy agency too much control over government computer networks. The security agency's interest in taking over the dominant role has met resistance, including the resignation of the Homeland Security Department official who was until last month in charge of coordinating cybersecurity efforts throughout the government. Rod Beckstrom, who resigned in March as director of the National Cyber Security Center at the Homeland Security Department, said ... that he feared that the N.S.A.'s push for a greater role in guarding the government's computer systems could give it the power to collect and analyze every e-mail message, text message and Google search conducted by every employee in every federal agency. Mr. Beckstrom said he believed that an intelligence service that is supposed to focus on foreign targets should not be given so much control over the flow of information within the United States government. To detect threats against the computer infrastructure - including hackers, viruses and intrusions by foreign agents and terrorists - cybersecurity guardians must have virtually unlimited access to networks. Mr. Beckstrom argues that those responsibilities should be divided among agencies. I have very serious concerns about the concentration of too much power in one agency," he said. "Power over information is so important, and it is so difficult to monitor, that we need to have checks and balances."