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The interior of a warehouse at Sagami General Depot, that was the site of Aug. 24, 2015’s early morning explosion at the U.S. Army base near Tokyo. Tanks of oxygen and compressed air exploded around 12:45 am setting the building on fire. U.S. Army and Japanese Defense Ministry officials concluded, in a Nov. 1, 2016 report, that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire.

The interior of a warehouse at Sagami General Depot, that was the site of Aug. 24, 2015’s early morning explosion at the U.S. Army base near Tokyo. Tanks of oxygen and compressed air exploded around 12:45 am setting the building on fire. U.S. Army and Japanese Defense Ministry officials concluded, in a Nov. 1, 2016 report, that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire. (James Kimber/Stars and Stripes)

The interior of a warehouse at Sagami General Depot, that was the site of Aug. 24, 2015’s early morning explosion at the U.S. Army base near Tokyo. Tanks of oxygen and compressed air exploded around 12:45 am setting the building on fire. U.S. Army and Japanese Defense Ministry officials concluded, in a Nov. 1, 2016 report, that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire.

The interior of a warehouse at Sagami General Depot, that was the site of Aug. 24, 2015’s early morning explosion at the U.S. Army base near Tokyo. Tanks of oxygen and compressed air exploded around 12:45 am setting the building on fire. U.S. Army and Japanese Defense Ministry officials concluded, in a Nov. 1, 2016 report, that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire. (James Kimber/Stars and Stripes)

The door closest to an Aug. 24, 2015 fire in a warehouse at Sagami General Depot contained much of the damage. U.S. Army and Japanese Defense Ministry officials concluded in a Nov. 1, 2016 report that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire.

The door closest to an Aug. 24, 2015 fire in a warehouse at Sagami General Depot contained much of the damage. U.S. Army and Japanese Defense Ministry officials concluded in a Nov. 1, 2016 report that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire. (James Kimber/Stars and Stripes)

The remains of a storage facility at the U.S. Army's Sagami Depot outside Tokyo are seen Aug. 24, 2015, after a fire burned for hours earlier that morning. U.S. Army and Japanese Defense Ministry officials concluded in a Nov. 1, 2016 report that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire.

The remains of a storage facility at the U.S. Army's Sagami Depot outside Tokyo are seen Aug. 24, 2015, after a fire burned for hours earlier that morning. U.S. Army and Japanese Defense Ministry officials concluded in a Nov. 1, 2016 report that a faulty valve or gasket on an oxygen tank stored in the facility was the most likely cause of the fire. (James Kimber/Stars and Stripes)

YOKOTA AIR BASE, Japan — A faulty valve or leaking gasket on an oxygen tank likely caused a warehouse explosion at an Army base near Tokyo last year, investigators say.

A report released last week by Army officials who looked into the August 2015 blaze at Sagami Depot says investigators couldn’t pin down a firm cause for the incident, said a statement from Sagamihara city, which received a copy of the report.

However, investigators ruled out arson or intentional destruction due to security measures at the installation, home of the 35th Combat Sustainment Support Battalion, the statement said.

It’s possible that oxygen leaking from a defective tank vibrated its valve, causing a spark that ignited material such as dust in the warehouse and heated other tanks, which exploded, one after another, the statement said.

The 12:45 a.m. explosion produced fireballs and streaking flames visible far from the depot in Sagamihara city, which is 34 miles from Tokyo Station. The fire smoldered for more than six hours before dying out, Army and Japanese fire officials said.

Officials had thought the warehouse contained Freon and nitrogen, but that turned out not to be the case.

The fire was allowed to burn out on its own because firefighters couldn’t be sure of the building’s contents and whether it would be safe to use water on the blaze.

Records listing the contents of warehouses at the depot have been updated, the statement said. Since the explosion, the Army removed all oxygen tanks from the depot.

The facility eventually will store oxygen again, but the number of tanks will be reduced, the statement said. Base personnel will conduct regular checks to prevent a reoccurrence.

Sagamihara Mayor Toshio Kayama said in a statement that it is regrettable the investigation took so long and that a firm cause of the fire wasn’t identified.

He requested the U.S. military take preventative measures to prevent future incidents.

Stripes staffer Hana Kusumoto contributed to this report.

cook.leon@stripes.com


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