Hiram Navarro had reasons for hope and despair Friday as rescuers hunted for his brother and dozens of others who were feared to be in a Mexico City office building that collapsed in this week’s magnitude-7.1 earthquake.

The good news, rescuers told Navarro and other relatives of the missing: Heat-sensing equipment indicates people may be alive in pockets of space amid the rubble of what had been a seven-story building.

The bad: Searchers have to move slowly and delicately, because the wreckage is unstable and could collapse further.

Relatives estimate up to 50 people were in the building along Avenida Alvaro Obregon in west-central Mexico City when the quake hit Tuesday. Family members have waited nearby ever since — some of them staying overnight in tents — monitoring the search and hoping for the best.

Similar scenes have been playing out across central Mexico as volunteers joined trained search and rescue workers to try to reach possible survivors and clear rubble, days after the quake that officials say killed at least 286 people.

Navarro, who believes his brother, Jesus, was in the building at Avenida Alvaro Obregon, said rescuers told relatives that they haven’t made contact with anyone inside, and they’re approaching the site cautiously.

“If we make one wrong move, the (pockets of space where people might be) will collapse,” Narvarro said, citing what rescuers told him.

Search to continue as long as there is hope

The earthquake turned dozens of buildings in central Mexico into dust and debris.

The temblor was the second major one to hit the country in less than two weeks, following a magnitude-8.1 earthquake farther south on September 8, killing nearly 100 people. Tuesday’s quake hit hours after a citywide drill on the anniversary of the 1985 earthquake that killed an estimated 9,500 people in and around Mexico City.

Search and rescue efforts could last “for at least two more weeks,” Luis Felipe Puente, Mexico’s civil protection coordinator, told CNN affiliate Foro TV on Friday.

“Our first phase is rescue and humanitarian aid,” Puente said. “Until we are absolutely certain that there are no more people missing, we will continue our search and rescue mission.”

‘We brought wheelbarrows, shovels, mallets’

Many rescue operations continued Friday, including at a Mexico City elementary school where 25 people — including 19 children — had died. Mexican navy officials monitoring the operation walked back earlier reports that rescuers had made contact with a girl trapped alive at the school, but they haven’t ruled out that someone else could be found in the debris.

Throughout the region, buses carried volunteers from Mexico and beyond to disaster sites, where they bolstered search-and-rescue efforts. People formed human chains to pass along supplies and remove chunks of lumber and concrete.

Volunteer Jorge Carreño left Mexico City for the small town of Jojutla, where some homes had collapsed. There, human chains spanning entire blocks worked Thursday, carrying buckets full of debris away from collapsed homes.

“We brought wheelbarrows, shovels, mallets. Everybody has hard hats, gloves and is ready to help,” Carreño told CNN.

President Enrique Peña Nieto declared a national emergency, and the country is observing three days of national mourning. An unaccounted number of people are staying at shelters around the capital after losing their homes. Schools have closed indefinitely, and millions initially were without power.

In all, 148 deaths were reported in Mexico City, one of North America’s most populous metropolises with more than 21 million people. Elsewhere, 73 deaths were reported in Morelos state, 45 in Puebla state, 13 in the state of Mexico, six in Guerrero state and one in Oaxaca state.