A Bangladesh Air Force F-7 trainer jet crashed into Milestone College in the capital, Dhaka, on Monday, killing at least 31 people and injuring more than 170, according to the military’s public relations division.

The dead include at least 25 children, one teacher and the pilot.

The initial death toll was 22, but the number rose after several people succumbed to injuries at surrounding hospitals, ISPR said in an updated statement.

Chief Adviser Muhammad Yunus, who holds a role similar to prime minister, mourned the loss and pledged a formal investigation.

“I express my deep grief and sorrow over the tragic incident of casualties caused by the crash of a Bangladesh Air Force F-7 BJI training aircraft at the Milestone School and College campus in the Diabari area of the capital today,” Yunus said in a statement. “The loss suffered by the Air Force, the students, parents, teachers, and staff of Milestone School and College, as well as others affected by this accident, is irreparable. This is a moment of profound pain for the nation.”

Yunus declared a national day of mourning Tuesday.

Local media reported that the jet slammed into a multi-story building that housed third- and fourth-grade classrooms shortly before 1:30 p.m. A large explosion followed the crash, igniting surrounding areas in flames.

Hundreds of students staged a protest around the school on Tuesday, demanding answers and an accurate accounting of the dead. A local television station reported at least 80 students were injured in clashes with riot police.

According to Reuters, protesters chanted: “Why did our brothers die? We demand answers!”

“The exact number of people killed and injured must be made public,” one student told The Associated Press.

The crash and protest come nearly a year after mass student-led demonstrations over unemployment rocked the country. Then-Prime Minister Sheikh Hasina was ousted and fled the country to India in August.