UPDATE: April 18, 2016, 2:21 p.m. EDT -- In a message posted on Twitter, Israeli police said the bus explosion in Jerusalem injured 21 people; two were in serious condition, six were moderately injured and eight lightly injured.

UPDATE: April 18, 2016, 1:15 p.m. EDT: Jerusalem Mayor Nir Barkat told local media that the bus blast in a southwestern neighborbood of the city was caused by a bomb in the undercarriage of the vehicle. Israel's police chief wouldn't elaborate on who was responsible, but he confirmed the explosion was the result of a bombing, reportedly an Improvised Explosive Device (IED). Two of the 15 casualties have not been identified, leading to speculation that they are the bombers responsible for the attack. This has yet to be confirmed by Israeli police.

Original story:

TEL AVIV, Israel — At least 15 people have been injured in a bus explosion in southwest Jerusalem.

Israeli ambulance rescue service Magen David Adom reported the explosion Monday and said at least 15 people had been wounded, including two critically.

Initial reports by the Israeli police suggested that the explosion was the result of a terror attack, but bystanders said the bus was empty, prompting speculation that it was a case of mechanical failure.

Scenes from the explosion, published on Twitter by Israeli police spokesman Micky Rosenfeld, revealed billowing smoke and a huge fire emanating from a burned-out bus.

Local police report "all avenues are being investigated."

This is a developing story. Check back here for updates.