BEIRUT — They had been hoping for a miracle Friday as Lebanese marked a month since the massive explosion that destroyed much of their capital.

A possible survivor of the Aug. 4 blast had been located a day earlier, against all odds, under the rubble of a collapsed building. Sensors had detected what might be a pulse. Rescue workers said their specialized equipment suggested the pulse might belong to a child, curled up alongside the body of an adult who was almost certainly dead.

But as the rescue effort slowed to a crawl Friday night, those hopes began to fade. The TV cameras that had been broadcasting live coverage of the operation were packed up and taken away. Onlookers drifted off.

The electricity went out, and the eerily uninhabited streets were plunged into darkness save for the illumination of the search operation's floodlights, pinpointing the spot in the ruins of a once elegant, pink four-story house where a sniffer dog belonging to a Chilean rescue team had alerted his handlers to possible signs of life two days before.

"I wanted to see if there would be one last sign of hope," said Maya Mansour, 30, as she gave up waiting and headed off to say goodbye to friends before leaving Lebanon for good on Saturday, among a surge of Lebanese who are emigrating in the wake of the disaster.

"It would have been something to grab onto before I leave," she said, looking disappointed.

The news that there may yet be survivors to be rescued had captivated a city plunged into despair by the death and devastation inflicted by the explosion, which killed nearly 200 people, injured 6,000 and wiped out some of the city's most historical and vibrant neighborhoods.

Television channels broadcast live as cranes and bulldozers arrived and departed amid clouds of dust. The graceful arches of one of the city's traditional old houses came into view as more and more rubble was shifted onto the street.

The pulse detected by sensors Thursday was coming at the rate of 18 beats per second — if a heartbeat, enough to sustain human life, but barely, according to cardiologist Nabil Khoury who was among those gripped by the search. "I want to believe," he said.

On Friday morning, the rescue workers said the pulse was still being detected, but at a rate of just 7 beats per second. By Friday night, rescue teams declined to discuss whether any pulse was still being detected. "Out of respect for people and their families, the rescue team prefers not to share information about the pulses," a leader in the Chilean team, Topos, told reporters shortly before the effort was called off.

It may turn out that there never was anyone left alive under the wrecked house, or that there weren't any people buried there at all.

But the incident also raises questions about the rigor of the Lebanese authorities' search-and-rescue operations in the wake of the blast. The destroyed house lies on a well-trodden road and has been photographed and Instagrammed multiple times by Beirutis as one of the more vivid examples of the devastation.

It emerged that no one had ever checked to see whether people might have been buried there — further evidence, activists said, of the kind of negligence that had allowed 2,700 tons of abandoned ammonium nitrate to sit unattended at Beirut's port for years until it was ignited in a still-unexplained fire.

Residents in the area had told the authorities multiple times they had detected the smell of decomposing bodies emanating from the ruins, but no one had ever made the effort to check, said Melissa Fadlallah, a Beirut activist. She was who was among a group of protesters who convinced the rescuers to resume the search overnight Thursday after the Lebanese authorities initially suspended the operation.

"Why did it take a Chilean dog and Chilean technology? Why did it take a month to check the building?" she asked. "How embarrassing is that?"

A French team reported that their sniffer dog had detected life at the scene last week but no one followed through, said Edward Bitar of the nongovernmental organization Live Love Lebanon, which was helping the rescue effort.

The effort was complicated by disputes among the different agencies involved in the search. The Chilean team, renowned worldwide for having rescued a survivor after 27 days in the wake of the Haitian earthquake, appeared confident that there was still hope that survivors could be found. The Chilean team leader said his team's sensors were 80 percent accurate.

A Lebanese leader of one of the agencies involved in the rescue operation was skeptical. He said he had called off his men because the search had revealed no signs of life. "There's nothing, there's nothing, there's nothing," said the team leader, who spoke on the condition of anonymity because of the sensitivity of the subject.