JAKARTA (AFP) - An explosion in the centre of the Indonesian capital Jakarta injured four people on Wednesday, one of them seriously, police said.
They declined to say whether a bomb caused the explosion but said dozens of packages containing nails and "suspicious" black powder were found nearby.
The blast occurred in the early afternoon in a densely populated residential area near Tanah Abang.
One man suffered serious burns from the blast and wounds from flying nails, while three others were slightly hurt.
Jakarta police chief Unggung Cahyono said officers found nearly 50 bags near the blast zone containing items deemed "suspicious".
"It's like a firecracker, containing suspicious black powder, nails and stones," he told broadcaster TV One.
"It is possible that at the time of assembling, there might be a technical error which caused the explosion."
However the police chief said a bomb normally required a timer and a detonator, neither of which were discovered at the scene, though police were still investigating all options.
The three slightly injured men are expected to be questioned soon, he added.
A crude chlorine bomb failed to detonate properly in a Jakarta shopping mall in February. Policed later linked it to Indonesian militants believed to have returned from fighting with the Islamic State in Iraq and Syria (ISIS).
Indonesia has waged a crackdown on terror groups for more than 15 years. There have been deadly attacks on Western and other targets, including the 2002 Bali bombings which killed 202 people.
The crackdown has been credited with weakening key networks.