A 25-ton crane hoisted cracked concrete slabs off an earthquake-leveled apartment building today in a last, desperate effort to rescue a 9-year-old boy still believed by some to be trapped alive inside.

Weary rescue workers who had scooped out rubble by bucket brigade, watched from the sidewalk, their labors interrupted for the first time by machines.

"We have exhausted all our other possibilities," said Luis Maldonado Diaz, the uncle of Luis Ramon Navarrete, the boy who was caught in the building's wreckage as it collapsed the morning of Sept. 19.

"We think the decision to use heavy machinery is correct," he said, speaking for the family.

His tone was resigned. At first, relatives and rescue workers opposed the introduction of heavy equipment, preferring to trust in the patient excavation by volunteer miners. But for 16 days now, Maldonado and a steadily expanding search crew had been struggling unsuccessfully to get through to the smashed apartment containing his father, Luis Maldonado Sr., believed to be dead, and his nephew. Many have indicated skepticism at reported contacts with the child as well as at the prospects of his surviving for such a long time.

"We simply are running out of energy, out of resources," Maldonado said. "We can't continue to risk the lives" of the rescue workers, he added.

Today at dawn, the latest tunnel being burrowed toward the boy collapsed, workers reported. Jesus Rangel, a civil engineer appointed by the city to supervise the rescue crew, said the tunneling miners already had broken through to the spot where they thought Luis Ramon lay trapped. The boy was not there, Rangel said.

Asked directly today if there is any assurance that the boy remains alive, Mexico City Police Chief Gen. Ramon Mota Sanchez said, "No, there is not. But that is not a judgment for us to make."

Likening the rescue effort to a bomb scare where "authorities are forced to respond whether or not the alarm is false," Mota acknowledged that the boy's chances to be saved are waning quickly. "Every minute now is counted against us," he said.

For three days now, millions of Mexicans have been anticipating the boy's rescue as if it were the cathartic last act of a national tragedy. The capital's sensational afternoon newspapers repeatedly headlined unsubstantiated accounts of conversations with the child; the guarded optimism of rescue workers was distorted into announcements of imminent breakthroughs.

Reports of the rescue effort's progress became grist for radio bulletins, replacing the dreary accumulation of official statistics that have been quantifying the earthquake's toll: 412 buildings toppled, another 300 ready to fall, $2.5 billion in damage, 8,000 bodies recovered and thousands of people still reported missing.

"We have to hope," said Bruno Rivas, a resident of the colonial-era neighborhood who had worked clearing rubble from the building in the search effort's first week.

But Rivas acknowledged the grim likelihood that the boy will be found dead.

Like several other neighbors and volunteers, Rivas insisted that in the days immediately after the quake he heard the "faint screaming of a child" in the ruins.

Reports of the screams began attracting more volunteers until the apartment building at 148 Venustiano Carranza St. became the site of the last and one of the largest of the city's postearthquake rescue operations.

But it has been more than a week since anyone has claimed to hear the child's cries. Some rescue workers yesterday believed they heard a muffled "yes" when they shouted in the tunnel, asking if the boy was there. Other volunteers, inside at the same time, however, said they could detect no response at all.

"After a while, when you're working in the dark without sleep, it's hard to tell if you are hearing things or not," one worker said. Periodically, the rescue workers demand total silence as they attempt to pick up sounds and vibrations with high-powered microphones and portable seismographs. The latest of a series of encouraging reports from these devices came at 4 this morning, when volunteer Samuel Guerrero said his National University rescue team seismograph detected "a clear response" to their tapped signals.

Mota, however, urged caution in reading too much into the listening devices.

"There have been indications of movement, but they could be the sound of falling rocks and debris," he said.