Story highlights Israeli president and Prime Minister call it a terrorist attack

At least two of the victims are seriously wounded

Jerusalem (CNN) At least 21 people were hurt Monday when an explosive device went off inside a city bus in Jerusalem, Israeli police and officials said, causing a large fire that spread to an adjacent bus and damaged at least one car.

Israel's president and Prime Minister called it a terrorist attack. Police were more cautious, saying that while it was a deliberate attack, they were looking at all possibilities, including that it was a criminal act.

All of those hurt were on the bus that contained the explosive device, which was on the back of the bus, Jerusalem District Police Commander Yoram HaLevi told CNN affiliate Ch.2 Israel.

At least two of the victims were seriously wounded.

"We will discover who prepared this explosive charge, we will reach those who sent them and those behind them, we will hold these terrorists accountable," Prime Minister Benjamin Netanyahu said.

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