Even as the police locked down a large area of the city after the blasts, the suspect, dressed as a police officer, entered the youth camp on the island of Utoya, about 19 miles northwest of Oslo, a Norwegian security official said, and opened fire. “He said it was a routine check in connection with the terror attack in Oslo,” one witness told VG Nett, the Web site of a national newspaper.

Of the at least 80 people killed on the island, some were as young as 16, the police said on national television early Saturday.

Terrified youths jumped into the water to escape. “Kids have started to swim in a panic, and Utoya is far from the mainland,” said Bjorn Jarle Roberg-Larsen, a Labor Party member who spoke by phone with teenagers on the island, which has no bridge to the mainland. “Others are hiding. Those I spoke with don’t want to talk more. They’re scared to death.”

Many could not flee in time.

“He first shot people on the island,” a 15-year-old camper named Elise told The Associated Press. “Afterward he started shooting people in the water.”