WASHINGTON  Leading Democrats on the House Judiciary Committee on Thursday reacted skeptically to the idea of overhauling surveillance laws to make it easier to wiretap people who communicate online rather than by telephone, a major priority for the Federal Bureau of Investigation.

The lawmakers expressed their concerns at a House hearing devoted to a problem that law enforcement officials call “going dark”  investigators’ inability to carry out court-approved wiretap orders when the people who are the targets communicate using services that lack a surveillance capability.

The F.B.I. has been quietly laying the groundwork for years for a push to require Internet-based communications services  like Gmail, Facebook, Twitter, BlackBerry and Skype  to design their systems with a built-in way to comply with wiretap orders. On Thursday, the bureau made its first full airing of the “going dark” problem.

“Due to the revolutionary expansion of communications technology in recent years, the government finds that it is rapidly losing ground in its ability to execute court orders with respect to Internet-based communications,” said the F.B.I.’s general counsel, Valerie Caproni.