The death toll from a building collapse in the Miami suburb of Surfside rose to 11 on Monday, as rescue operations continued into a fifth day. Officials urged people to maintain hope survivors might be found in the wreckage of the collapsed condo building. A further 150 people who may have been in the building remained unaccounted for, the mayor of Miami-Dade, Daniella Levine Cava, said on Monday evening. No survivors have been found since Thursday, and rescue teams have been conducting round-the-clock searches of the Champlain Towers South. “They are out there with every resource that they need to ensure they can search this area,” Levine Cava said. “They can sweep the mound with dogs, with cameras and sonar.” The mayor said that the numbers of deceased and missing people were still “very fluid”. Andy Alvarez, a deputy incident commander with Miami-Dade fire rescue, told ABC on Monday rescuers were still hopeful. “This is a frantic search to continue to see that hope, that miracle, to see who we can bring out of this building alive,” Alvarez said. Alvarez said rescue teams found some voids in the wreckage, mostly in the basement and near the parking garage. “We have over 80 rescuers at a time that are breaching the walls that collapsed, in a frantic effort to try to rescue those that are still viable and to get to those voids that we typically know exist in these buildings,” Alvarez said. Alvarez said conditions were “not ideal” because of humidity, rain and heat. “We’re holding up because we’re all holding up for that hope, that faith that we are going to be able to rescue somebody,” Alvarez said. “We are working tirelessly to try to bring victims that are underneath that rubble and rescue them.” The search includes more than 300 emergency personnel, international experts and equipment including underground sonar systems. Efforts are also under way to determine the cause of the collapse. Structural engineers have identified several theories, but determining the cause could take months or years. In 2018, an engineer flagged concerns about “major structural damage” at the tower but the report did not indicate the building was at risk of collapse. The engineering report, dated 8 October 2018, appeared to have been discussed at a board meeting of the Champlain Tower South Condominium Association the next month. At the November meeting, a Surfside building official said “the building is in very good shape”, according to meeting minutes obtained by NPR. The building collapsed at about 1.30am on Thursday. No survivors have been found since the first hours following the collapse. Rescuers dug a trench 125ft long, 20ft across and 40ft deep overnight on Saturday and found remains but no survivors. Families of the missing traveled by bus on Sunday to a site where they could watch rescue teams dig through the rubble and move large sections with cranes. An official who accompanied the families told the Miami-Herald several people yelled the names of their loved ones in the hopes they might hear. Dianne Ohayon, whose parents, Myriam and Arnie Notkin, were in the building, told the Associated Press: “We are just waiting for answers. That’s what we want. It’s hard to go through these long days and we haven’t gotten any answers yet.” Alan Cominsky, the chief of the Miami-Dade fire rescue department, emphasized the operation must be slow and careful to protect rescuers as well as any survivors. “We can’t just go in and move things erratically, because that’s going to have the worst outcome possible,” Cominsky said on Sunday. At a White House press briefing on Monday, the press secretary, Jen Psaki, said Joe Biden wanted the federal government involved with an investigation of the collapse. Officials from several federal bodies, including the FBI and Occupational Safety and Health Administration (Osha), have been deployed to the scene. “We want to play any constructive role we can play with federal resources in getting to the bottom of it and preventing it from happening in the future,” Psaki said. Surfside has hired a structural engineer, Allyn Kilsheimer, who examined the Pentagon after it was attacked on 9/11, to study the tower collapse and the condition of nearby buildings. Kilsheimer, founder and chief executive of KCE Structural Engineers, has toured the site and “saw no visible evidence of any major structural concern”, according to the town. The Champlain Towers North condominium association has told the town it will hire its own structural engineering firm to investigate. The latest four victims identified by police were Christina Beatriz Elvira, 74, Leon Oliwkowicz, 80, Anna Ortiz, 46, and Luis Bermúdez, 26. Bermúdez lived on the seventh floor of the building with his mother, Ortiz. His father, also named Luis Bermúdez, wrote a tribute to his son on Facebook: “God decided that he wanted one more angel in heaven. I still do not believe it. I LOVE you and will love you forever.” Rescuers are using bucket brigades and heavy machinery atop a precarious mound of pulverized concrete, twisted steel and the remnants of dozens of households. “Every time there’s an action, there’s a reaction,” Miami-Dade assistant fire chief Raide Jadallah said during a news conference of efforts to shift the rubble. “It’s not an issue of we could just attach a couple of cords to a concrete boulder and lift it and call it a day.” Underscoring the dangerous nature of the work, he noted that families who rode buses to visit the site on Sunday witnessed a rescuer tumble 25ft down the pile. On Monday, a crane lifted a large slab of concrete from the debris pile, enabling about 30 rescuers in hard hats to move in and carry smaller pieces of debris into red buckets, which are emptied into a larger bin for a crane to remove. Jimmy Patronis, Florida’s chief financial officer and state fire marshal, said it was the largest deployment of such resources in Florida history that was not due to a hurricane.