JERUSALEM (AP) — A bus exploded in Jerusalem on Monday, wounding at least 15 people in what police said was a "terror attack," raising fears of a return to the Palestinian suicide attacks that ravaged Israeli cities a decade ago.

"There is no doubt that this was a terror attack," Jerusalem police commissioner Yoram Halevy said. He said 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.

Police spokesman Micky Rosenfeld said two people on board the bus that exploded were seriously wounded, while the other wounded people had been in a nearby bus and car that were also damaged.

It was not clear how many people were on the bus at the time it exploded. Police said the blast was caused by an explosive device detonated at the back of the bus.