Putting to rest any suspicion that the sightings at Heathrow, about 15 miles west of central London, were false alarms, Commander Cundy said, “Police officers were amongst those who saw the drone and a full criminal investigation has been launched.”

The Metropolitan Police, which investigates cases believed to involve terrorism throughout Britain, played a secondary role behind the Sussex Police in investigating the Gatwick case, because officials never determined whether it might be terrorism. That prompted calls from some lawmakers for the Metropolitan force to take over the investigation, particularly after two suspects were arrested by the Sussex Police, but then cleared and exonerated.

Operating a drone within a kilometer of an airport is illegal, but aviation security experts have called for an exclusion radius of five kilometers, or about three miles.

The partial shutdown on Tuesday was all the more dispiriting because it came only days after Heathrow and Gatwick announced they had ordered military-grade anti-drone equipment to ward off incursions. Both airports have declined to say what the new technology is, or what anti-drone measures they already had in place.

A spokeswoman for Gatwick told British news outlets that its new system had been installed and that it matched the capability of the military, which deployed at the airport during the shutdown there. The airport said it had spent several million pounds to purchase the equipment.

It was not clear whether Heathrow had already installed its new defense system. The airport was said by a person briefed on its security to have some limited anti-drone technology in place before the Gatwick incursions.

A BBC producer, speaking on Tuesday from a plane that was waiting to take off from Heathrow, told the network that passengers on her flight had not been given a reason for the delay. “At the moment,” she said, “we’ve been told we’re not going anywhere.”