At least 84 people have been confirmed dead in the shooting spree at a youth camp on Utoeya island, Norwegian police said on Saturday.
Officers said the killings were of “catastrophic dimensions.”
The 32-year-old suspected gunman is Norwegian. He entered the camp disguised as a police officer before opening fire.
Some of the teenagers tried to escape by diving into the water.
Officials said the gunman has links to extreme far-right groups and appears to have acted alone.
Detectives are linking the shooting with a bomb blast two hours earlier that rocked Oslo’s government district.
At least seven people died in the bombing and several dozens were injured.
Police say they fear the death toll could rise even further.
It is the biggest attack in western Europe since the 2005 London tube bombings that claimed 52 lives.