An “apparent miscalculation” of the weight of fireworks detonated by police is suspected in the explosion that shook a South Los Angeles neighborhood late last month, according to preliminary findings released Monday.

What was meant to be a safe detonation of homemade fireworks ended in an explosion that injured 17 people on June 30 in the 700 block of East 27th Street, where Los Angeles Police Department officers had spent the day seizing a large cache of fireworks from a home.

Officials said police opted to detonate some fireworks that were “leaking” and deemed to be too unstable to move. They loaded the explosive material into a total containment vessel with an iron chamber designed to entirely contain the explosion, according to LAPD.

The officers estimated they were putting in 16.5 pounds of explosives into the vessel, which is designed to take a maximum capacity of 25 pounds for a one-time detonation, LAPD Chief Michel Moore said in a news conference Monday.

But investigators later weighed the remains and calculated that just over 42 pounds of net explosive weight was in the vessel, Moore said.

“Based on information gathered at this point, it is believed that the net explosive weight placed into the [vessel] exceeded its rated capacity,” Moore said.

Moore said the department believes “human error” led to the miscalculation of the amount of material going into the vessel, explaining that the LAPD team calculated the weight of the explosives by slicing into two firework samples, extracting material from inside, and estimating the weight — not by using a scale.

LAPD protocols allow for a physical and visual estimation of the weight of explosive materials, according to the chief.

Another theory is that the vessel or some of its components failed, according to Michael Hoffman, assistant special agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives in Los Angeles.

The investigation is still ongoing. A specialist is looking for any anomalies or deficiencies in the containment vessel and its components, according to Hoffman.

The bomb squad vessel that exploded had been in service for more than a decade, with the June 30 detonation being the 42nd time it was used.

The LAPD supervisor and bomb technicians involved in the detonation have been removed from the field, Moore said.

“If mistakes are made in regards to established protocols, I’ll hold the appropriate individuals accountable,” the chief said.

Moore apologized to residents affected by the blast, which injured civilians and damaged nearby homes and vehicles.

“I want to personally express my apologies to every resident, business operator and customer that was dramatically impacted by this incident … I acknowledge that there are many more individuals who were traumatized by the explosion itself,” the chief said.

The explosion, which police have previously described as “completely unexpected,” displaced residents from their homes for more than a week as investigators probed the blast site.

It raised questions over what went wrong and why the LAPD detonated the fireworks in a populated neighborhood.

Also, with six of the 17 injured being civilians, several residents have questioned why there were still people nearby, despite LAPD saying they went door to door telling people to evacuate.

L.A. City Councilman Curren Price released a statement saying he was “infuriated” by the preliminary findings, calling what happened an “act of negligence.”

“This is by far one of the LAPD’s largest blunders in recent history, which has further betrayed the trust of our South LA community,” Price said in a statement. “My constituents feel that this was a blatant disregard for their safety and our community of color. This tragedy could have been prevented and I wholeheartedly believe that additional safety precautions would have been taken in a more affluent community.”

The Police Department is now reviewing its protocols on the detonation of explosives.

“We have taken significant steps to strengthen operating protocols, documentation, oversight of any destruction of fireworks and other explosive materials in the city, and more work remains in this aspect of this investigation,” he said.