The Brussels attackers used ammonium nitrate bombs, two sources tell NBC News. The explosives were estimated at 44 pounds each.
During a raid on a residence linked to the suspects, officials found other bomb components, the Belgian prosecutor's office said on Wednesday.
More than 30 pounds of another high explosive known as TATP was found along with 150 liters of acetone and 30 liters of hydrogen peroxide, both chemicals that can be used to make bombs.
At that location in the Schaerbeek neighborhood, investigators also seized detonators, a suitcase full of nails and screws, plastic tubs, ventilators and glass utensils — all signs the home was being used as a bomb-making factory.
It was not clear if the terrorists were in the process of planning a bigger attack and decided to move sooner, perhaps after the arrest of a key suspect in the Paris attacks, Salah Abdeslam, a senior U.S. counter-terrorism official said.
Or they could have been planning a series of attacks "and yesterday was just the start of that plan," the official said.
"We don't know why they had so much material," the official added.
The suspects brought three suitcase-style bombs to the Brussels Airport on Tuesday morning. They were captured on security video pushing luggage carts with large black bags through the busy terminal.
They blew up the first two bombs 10 seconds apart, officials said.
One of the terrorists dropped the third bag, which contained the largest explosive charge, and fled, the prosecutor's office said. It exploded as the bomb squad arrived, but no one was hurt, the prosecutor said.
The ammonium nitrate was detected with swipe tests, sources said. It is a relatively stable high explosive, detonating at a speed of more than 5,000 feet per second. It is used as a key component of ANFO (ammonium nitrate/fuel oil), a popular industrial explosive.
TATP can be used as a booster charge to ensure a powerful detonation of a fairly stable explosive like an ammonium nitrate bomb. It can be used in a mixture with ammonium nitrate or in compact explosive devices like suicide vests.
Ammonium nitrate was used in the 1995 Oklahoma City bombing and in the 1993 bombing of the World Trade Center. It's also a widely used fertilizer.
