...Four people were killed, the authorities said: three men and one woman, all of whom were working on the project. Three other people were injured, but none of their injuries were life-threatening, the authorities said.

Witnesses described hearing a loud booming sound around the time of the collapse. One witness told RTVE News, Spain’s public broadcaster, that he heard “a tremendous crash” that sounded like a bomb exploding, but longer-lasting. Another witness said he felt a tremor, “like some kind of earthquake.”...

The police have opened an investigation into the incident, the authorities said. While the cause is still unknown, Mr. Martínez-Almeida said “it’s obvious that there was material piled up on the sixth floor, and therefore, it could have influenced the building’s collapse.”...

The structure underwent city inspections in 2002, 2012 and 2022. Its last two inspections were classed as “unfavorable” because of issues including the condition of the structure, facade, foundations and roof, as well as its plumbing and sanitation network.

In 2022, the building was purchased by RSR Singular Assets Europe, a real estate investor based in Saudi Arabia that owns properties across Spain and Portugal, according to the organization’s financial reports. The company, which purchased the building for 24.5 million euros, or roughly $29 million, had plans to turn it into a four-star hotel to be opened in 2026.

Rehbilita, a Madrid-based construction company with projects across the city, was in charge of the project, according to its website. RSR and Rehbilita did not respond to requests for comment.

The building was issued a construction license in February 2025, Mr. Martínez-Almeida said. He added that the city’s official channels had “no record of any complaints regarding the work being carried out on the building.” However, he said the authorities were conducting “a thorough review” to see if any complaints might have been overlooked.