It was about 1 p.m. on Monday when Tahmina Akhter arrived to pick her two children up from school. Her son, age 7, emerged a few minutes later, Ms. Akhter said, but went back to fetch his 12-year-old sister.

That must have been just after 1:18 p.m.

Moments later, before the siblings could return, a fighter jet flipped in midair and crashed into their school building in the Uttara neighborhood of Dhaka, the capital of Bangladesh. Its unspent fuel ignited like napalm. Brother and sister were both severely burned, along with dozens of other students.

Within the next two days, both had died.

The children were among at least 30 killed by the crash. The Bangladeshi military said in a statement on Monday that a mechanical error had caused the crash and that further investigation was underway.

Most of those killed on the ground were children between 9 and 14. The severe nature of the burns they suffered left at least 55 more in hospitals. Seven were in critical condition at the National Institute of Burn and Plastic Surgery on Thursday, including four on life support.

Dr. Anubhav Gupta, a plastic surgeon and burn specialist at Ganga Ram Hospital in New Delhi, said that for burn patients who survive the first two days after an accident, the next two or three days is “our golden time.” The patients have been stabilized by then, he said, with wounds dressed and intensive hydration supplied, and infection has not yet set in. It is the moment of opportunity for transplants of skin and artificial tissues.

There was no such opportunity for Ms. Akhter’s children. Soon after the plane crash, Ms. Akhter was joined by her husband, Ashraful Islam, an ex-soldier. He had been less than a mile away and rushed to the school in time to see a fireball billowing out.