Israeli authorities on Tuesday were investigating a Jerusalem bus bombing that wounded 21 people and raised fears of a sharp escalation in violence, while police planned tightened security for the Jewish Passover holiday.
The bomb tore through a bus in a relatively isolated area of southern Jerusalem on Monday night, with the fire spreading to a second bus.
In total at least 21 people were wounded, police said, with Israeli media reporting a 15-year-old girl in serious condition.
Israeli domestic security agency Shin Bet referred to the explosion as a "terror attack".
Police spokesman Micky Rosenfeld confirmed officers were seeking to question the wounded and did not rule out the possibility of potential suspects among them.
"The investigation is looking to see how the explosive device was placed on the bus," he said.
Police imposed a gag order on further details of the investigation and any suspects.
Prime Minister Benjamin Netanyahu promised to "find whoever prepared this explosive device".
"We'll settle the score with these terrorists."
The Palestinian Islamist group Hamas, which rules the Israeli-blockaded Gaza Strip, welcomed the attack but there was no claim of responsibility.
The explosion comes with tensions high following Israel's deadly use of force against Palestinian protesters in the occupied West Bank and Gaza Strip, killing 201 Palestinians and 28 Israelis since last October.
Bus bombings were common during the second Palestinian intifada, or uprising, in the early 2000s, but Monday's attack was the first bomb targeting a bus in Jerusalem since 2011, when a British tourist was killed.
In Tel Aviv, a bomb exploded on an empty bus in 2013 in what Israeli authorities called a "terrorist" attack.
But Avi Dichter, a lawmaker and former Shin Bet chief, stressed the bomb did far less damage than those during the intifada.
The frame of the bus was still largely intact, unlike with larger bombs.
"The explosive charge was much lower than those we experienced during the suicide bombings of the second intifada," Dichter told public radio.
The blast comes ahead of the Jewish Passover holiday, which starts on Friday evening.
The Israeli government regularly closes off access to Israel for Palestinians during major Jewish religious festivals, and is expected to do so again.
Rosenfeld said the threat level would be raised during the week-long holiday.
"Security assessments were made immediately yesterday evening following the attack," he told AFP.
"Extra police units and border police are patrolling public areas," including bus stations and the light rail tram system in the city, he said, without specifying how many reinforcements.
Thousands of Jewish pilgrims flock to Jerusalem and other holy sites during Passover, which commemorates the Jewish exodus in Biblical times.
Brachie Sprung, spokeswoman for Jerusalem Mayor Nir Barkat, said the city council was not planning any changes to its schedule.
"We are telling people to go back to normal -- we don't cancel events," she told AFP. "We talk about being super cautious but going back to normal as quickly as possible."
Itamar Yaar, former deputy head of Israel's National Security Council, said he did not expect any major security clampdown in the city.
"I really don't think there will be any change in the Israeli behaviour," Yaar told AFP. "Perhaps in the first few days you will see more checks at checkpoints.
"But the most effective activities in this case are collecting intelligence and trying to reach the people that organised it."
Separately on Tuesday, an Israeli court ruled that a Jewish man found to be the ringleader of the beating and burning alive of a Palestinian teenager in 2014 was sane and convicted him of the attack.
Israeli settler Yosef Haim Ben-David, 31, was found in November to have led the assault, but his lawyers had submitted last-minute documents claiming he suffered from mental illness.
*This story was edited by Ahram Online.