About 50 rescue workers and volunteers, including a specialist team from Chile, had yet to locate anyone after sensors on Thursday detected signs of breathing and heat. But they said they would continue while there was a small chance of finding a survivor, and had narrowed their search.
"Always in search operations like this, you can neither lose hope nor absolutely say there is hope," George Abou Moussa, director of operations in Lebanon's civil defense, told Reuters.
The Aug. 4 blast killed about 190 people, injured 6,000 more and devastated whole neighborhoods. The authorities held ceremonies on Friday to mark a month since the explosion tore into a city already reeling from a crippling economic crisis.
Rescue efforts dominated local and social media, as the Lebanese were transfixed, desperate for some good news.
The ruined building where the search was continuing lies between the residential districts of Gemmayze and Mar Mikhael, among the hardest hit areas by the blast and home to many old buildings that crumbled as the shockwave ripped through.
Work was slow, rescue workers said, as the badly damaged building was at risk of collapse.
"The building is really crumbling, it's scary and there's a lot of danger to the team," Abou Moussa said.
Workers were using shovels and their hands to dig, while mechanical diggers and a crane lifted heavy debris.
Emmanuel Durand, a French civil engineer who was training local university students, volunteered his services and was working with the rescuers to monitor the structure.
Scanning the building with high-precision lasers, Durand said his team had so far not found any signs of movement.