The botched police detonation of a stash of illegal fireworks in Los Angeles that injured 17 people this month was caused by officers dramatically underestimating the weight of the explosives they were blowing up, authorities have said. Days before July Fourth, the LAPD bomb squad responded to an anonymous tip that a resident was stockpiling illegal fireworks in their backyard. They packed some of the explosives into a containment chamber to let off a controlled explosion, but got their math very, very wrong. “We believe a human error went to the miscalculation of the amount of material going into that vessel,” LAPD Chief Michel Moore said Monday. “It’s believed that the net explosive weight placed into the TCV exceeded its rated capacity.” Moore said technicians believed there was 16.5 pounds of explosives in the vessel, but it’s now thought there was more than 42 pounds. The explosion damaged dozens of homes, businesses and vehicles in the neighborhood. Moore described it as a “catastrophic failure.”