A bus has exploded in Jerusalem, wounding at least 21 people in what police said was a terror attack, raising fears of a return to the Palestinian suicide bombings that ravaged Israeli cities a decade ago.

“There is no doubt that this was a terror attack,” said Jerusalem police commissioner Yoram Halevy. He added that it was too early to know the identity of the attacker or if it was a suicide bombing.

“We are investigating where the explosive device came from, who planted it, how it got on the bus. All this is in the initial stages of investigation,” he said.

Police officials had initially said all options were being examined, including a possible technical malfunction.

The blast came as jittery Israelis prepared for the Passover holiday amid a wave of Palestinian attacks, mostly stabbings.

Over the last seven months, 28 Israelis and two Americans have been killed in Palestinian attacks, while at least 189 Palestinians have been killed.

Israel says most of the Palestinians killed were attackers, with the rest killed in clashes with security forces.

The Magen David Adom rescue service said 15 people were wounded, two of them seriously, in the bus explosion.

The wounded were taken to Shaare Zedek Medical Center and the Hadassah Ein Karem Medical Center in the capital.