In the days before the explosion, people inside the building had been complaining about a bad smell coming from the basement. Store owners reported the odor to the San Juan Gas Company, which was owned at the time by Enron Corporation. The building itself had no gas service, which made the source of the smell mysterious and, in retrospect, all the more alarming. The NTSB investigation later determined that an underground propane gas pipe running beneath the building had cracked. Years earlier, a water main had been installed below the gas line, bending the already tightly bent pipe and adding stress until it broke. Propane, heavier than air, seeped into the building's basement and accumulated there, invisible and waiting. The ignition source turned out to be an air-conditioning switch with heated wiring.

At 8:35 a.m., the accumulated propane ignited. The blast tore through the building from below, collapsing floors and walls inward. Much of the interior caved in on the people inside. Others were killed in the streets surrounding the building. In the immediate aftermath, bodies were laid on the pavement in front of the nearby La Milagrosa church, where Cardinal Luis Aponte Martinez administered last rites. "There were just parts of bodies lying in the street, torsos, bones, cars blasted against the building," Police Chief Pedro Toledo told reporters. A nearby school, mercifully, suffered no casualties. The initial assumption was terrorism. Previous acts of political violence in Puerto Rico led investigators to suspect a bomb planted by clandestine paramilitaries, or arson. But no trace of explosives was found, no flammable accelerants. The destruction came from below the ground, from infrastructure that had failed while the people responsible for maintaining it denied there was any problem.

The San Juan Gas Company denied responsibility, claiming the building had no gas service at the time of the explosion. This was technically true and entirely beside the point. The propane pipe that broke ran beneath the building as part of the neighborhood's underground gas infrastructure, regardless of whether the building above was a customer. The pipe had been installed at a tight bend, already under stress. When the water main was added below it years later, the additional pressure was enough to crack it. The NTSB investigation, launched after President Bill Clinton declared Puerto Rico a disaster area, documented these cascading failures: poor pipe installation, subsequent infrastructure work that damaged existing lines, and a gas leak reported by civilians but never adequately investigated by the utility. Each failure alone might not have been fatal. Together, they killed 33 people who were simply going about their morning.

The destroyed building was demolished. According to a city resolution passed in 2007, all underground gas lines in Rio Piedras were subsequently removed to prevent a recurrence. A mural now marks the site where the Humberto Vidal building stood, a memorial to the 33 people who died there. The disaster was later examined in a 2005 episode of the National Geographic documentary series Seconds From Disaster. But for the families of the victims and the community of Rio Piedras, the explosion is not a documentary subject. It is the morning the phone rang, the day a familiar street became unrecognizable, the permanent absence of someone who had left the house to run an errand. The youngest victims were shoppers. The oldest were employees who had worked in the building for years. They are remembered not for how they died but for the ordinary lives they were living when a cracked pipe beneath their feet erased everything.