Pakistani security officials have issued a warning that terrorists may be planning to target school buses with magnetic bombs. A memo sent by the office of Islamabad’s senior superintendent of police on Wednesday called on schools to increase security and to check underneath buses and other vehicles carrying students, reported Sky News. The warning came a day after Taliban gunmen killed more than 130 children and nine teachers at an army-run school in Peshawar on Tuesday.

Witnesses said that gunmen began shooting indiscriminately upon entering the school Tuesday morning and within minutes gunned down a large number of students. A military spokesman said that more than 100 were injured in the attack, many from gunshot wounds. The attack is thought to have been the deadliest in Pakistan in recent years and has elicited widespread shock in a country where bloody attacks by insurgents are far from uncommon.

Two bomb blasts were also reported close to a girls college near Peshawar on Wednesday, though no group has yet taken responsibility for the attack. No casualties were reported from the blasts, according to local media reports.

Prime Minister Nawaz Sharif announced that the country would begin three days of official mourning on Wednesday. Mass funerals and prayer vigils were held for the victims, and many schools across the country were closed as a mark of respect, according to the BBC.

The attack has also put Pakistan’s neighbor India on alert, with police forces in India taking steps to enhance security on Wednesday. Police in Delhi issued a set of guidelines to schools and colleges to ensure that necessary security measures were in place. Security vans have also been positioned close to prominent schools, with additional police forces deployed to patrol the surrounding areas, reported DNA India.

Pakistani army chief, Gen. Raheel Sharif, also traveled to Afghanistan on Wednesday to meet with Afghani and NATO leaders to discuss routing the threat of Taliban insurgents in both countries. Gen. Sharif reportedly shared vital intelligence with Afghani President Ashraf Ghani, who assured the Pakistani delegation of his cooperation against the Taliban, according to the New York Times.