BEIRUT • Investigators began searching through the wreckage of Beirut’s port Wednesday for clues to the cause of the massive explosion that ripped across the Lebanese capital, and the government ordered port officials put under house arrest amid speculation that negligence was to blame.

The investigation is focusing on how 2,750 tons of ammonium nitrate, a highly explosive chemical used in fertilizers, came to be stored at the facility for six years, and why nothing was done about it.

International aid flights began to arrive as Lebanon’s leaders struggled to deal with the shocking aftermath of Tuesday’s blast, crippled by an economic crisis and facing a public that blames chronic mismanagement and corruption among the ruling elite for the disaster.

The explosion at the port killed at least 135 people and wounded about 5,000, Health Minister Hamad Hassan said. Hospitals were overwhelmed — one that was damaged in the blast had to evacuate all its patients to a nearby field for treatment.

Buildings were damaged for miles around the city, and Beirut’s governor said Wednesday that hundreds of thousands might not be able to return to their homes for months.

It was the worst, most destructive single explosion to strike Lebanon in a history filled with destruction, in a 1975-1990 civil war, conflicts with Israel and periodic terror attacks.

A U.S. Defense Department official and member of the U.S. intelligence community said there were no indications the massive explosion in Lebanon’s capital was the result of an attack by either a nation state or proxy forces.

Both individuals spoke under condition of anonymity because they were not authorized to discuss intelligence briefings publicly.

Fueling speculation that negligence was to blame for the accident, an official letter circulating online showed the head of the customs department had warned repeatedly over the years that the huge stockpile of ammonium nitrate stored in a hangar in the port was a danger, and asked judicial officials for a ruling on a way to remove it.

A drone picture Wednesday shows the scene of an explosion at the seaport of Beirut. A massive explosion rocked Beirut on Tuesday, flattening much of the city’s port, damaging buildings across the capital and sending a giant mushroom cloud into the sky. At least 135 people were killed and 5,000 injured.

the associated press