Blast took place in a park close to Indonesia’s presidential palace and police are investigating cause.

A suspected smoke grenade exploded on Tuesday in Indonesia‘s National Monument park, near the presidential palace in central Jakarta, injuring at least two military officials, the city’s top police official said.

President Joko Widodo was not at the palace at the time of the blast, a presidential spokesman said.

The cause of the explosion was not immediately clear.

The blast was suspected to have been caused by a smoke grenade, and two injured military officials had been taken to hospital, Jakarta’s police chief, Gatot Eddy Pramono, told a televised news conference.

The two injured officers were doing exercises in the park at the time, Jakarta military chief Eko Margiyono added.

Asked if the blast was the result of an attack, Pramono responded: “We haven’t concluded so, because we’re still investigating.”

Last month, six people were wounded after a 24-year-old university student blew himself up outside police headquarters in the Indonesian city of Medan.

That attack was connected with Jamaah Ansharut Daulah (JAD), an outlawed Indonesian group linked to the Islamic State of Iraq and the Levant (ISIL or ISIS) group that has been responsible for a series of attacks across the country.

In October, a suspected fighters stabbed and wounded former security minister Wiranto at a function to open a university building. Wiranto had to have surgery but has since been discharged.