At least three persons were injured in an explosion in an intercity train in Assam’s Udalguri district on Saturday evening, PTI reported, quoting railway and police officials. The number of injured persons could be up to 11, according to ANI.

The explosion took place in a coach of the Kamakhya-Dekargaon Intercity Express at the Harisinga railway station around 7.04 pm, a Northeast Frontier Railway spokesperson said. One of those injured was in a critical condition after sustaining a head injury, the spokesperson told The Times of India.

An investigation is under way. Authorities have yet to find out whether the explosion was cause by a grenade or an improvised explosive device, unidentified officials told PTI.

“The sound from the blast seems to be of a low-intensity improvised explosive device,” Director General of Police (civil defence) Pallab Bhattacharya told The Times of India. “Police officials have rushed to the spot and it is difficult to ascertain what kind of blast it was.”

More details are awaited.