WASHINGTON — State investigators listened in on more than 3 million phone conversations last year as local prosecutors sought a record number of wiretaps, mostly to investigate drug crimes.

As the federal government has focused its resources on national security investigations, the responsibility for drug investigations — the focus of 80% of wiretaps — has fallen to state and local authorities.

A decade ago, federal and state investigators sought about the same number of wiretaps. Last year, state prosecutors obtained nearly three times as many wiretap authorizations as their federal counterparts: 1,378 to 461, according to an annual report by the Administrative Office of the U.S. Courts.

Before tapping someone’s phone, prosecutors must persuade a judge there is probable cause to believe the person is breaking the law. No federal or state judge denied such a request last year. Of the more than 15,000 applications filed in the last decade, only five were denied.

Technological advances have made it easier for local investigators to tap telephones and cell phones.

“It can be done from a central switching station rather than climbing poles and messing with wires the way it was in the ’70s,” said Clifford Fishman, a former New York City drug prosecutor and a Catholic University law professor.