Twenty one people were injured, one critically, when a bomb ripped through an empty Egged bus in Jerusalem’s Talpiot neighborhood, spraying nails, bolts and shrapnel flying, and setting fire to a nearby bus.

Jerusalem District Commander Yoram HaLevy said the mayhem was caused by an explosive device placed in the rear of the bus.

Eight of the wounded were rushed to the Hadassah Ein Karem Hospital, including a 28-year-old woman who was reported to be in serious condition. Three others were described in moderate condition—a 25-year-old man, a 40-year-old man and a 30-year-old woman—with four individuals lightly wounded, all in their 40s.

Jerusalem’s Shaare Zedek Medical Center is treating 10 of the people wounded in the attack, according to a hospital spokesman.

Of these 10, one is in critical condition with burns all over his body. According to some reports, police believe that the critically burned individual may be the terrorist. Two others at the medical center are moderately wounded; the rest lightly hurt. One of the wounded is currently undergoing surgery.

“We are still in the initial stages of the investigation,” said HaLevy. “We’re trying to find out primarily where the explosive device came from and who placed it on the bus,” he said, adding that it should not take police much longer to identify who planted it.

Professor Avraham Rivkind, head of the trauma unit at the Hadassah Ein Karem Hospital, told Ynet news that some of the injuries were in line with those from past terror bombings in Jerusalem: “There are penetration wounds. We saw nails and bolts in the diagnostic images, and we removed them.”