Norway's justice minister confirmed that the suspected gunman is Norwegian, the BBC reported late this afternoon.
"A person has been arrested... I have been informed that he is a Norwegian," Justice Minister Knut Storberget said at a press conference. "It would not be fair of me to go into more detail about this," he added.
As many as 700 people were believed to be taking part in the summer camp — in Utoya, outside the capital — most of them teenagers aged between 14 and 18, according to Business Insider.
Labor spokesman Per Gunnar Dahl told the AP that a gunman dressed in a police uniform started shooting at youths assembled for the party's annual youth camp.
A man dressed as a policeman shot wildly into a crowd, "hitting many people," the VG, a daily newspaper reported on its website. Police said they believed the bombing and the shooting were connected. Norwegian media was reporting that one person had been arrested on the island.
Oslo police say that the gunman had previously been spotted in the capital prior to the explosions. "That news has many hoping that the attacks were the work of a lone terrorist and not a coordinated attack," according to a Slate liveblog of the unfolding Oslo attacks.
A Reuters witness said several army soldiers had taken up position around Oslo city center. With police advising people to evacuate central Oslo, apparently in fear of more attacks, Prime Minister Jens Stoltenberg told Norwegian television in a phone call that the situation was "very serious". He said that police had told him not to say where he was speaking from.
Prime Minister Stoltenberg had reportedly been due to attend the event. His office was one of those hit by one of several explosions hours earlier, although he was reportedly working from home. At least 7 were killed in the bombing.
It wasn't immediately clear if the attacks were related.
Oslo police chief Anstein Gjengdal said anti-terror units were being sent to Utoya.
According to Reuters:
Bombings in city centers and marauding guns attacks are both tactics that have been used by Islamist militant gangs in recent years, mostly notably by al Qaeda-aligned guerrilla groups in Asia.
However, citing London-based Exclusive Analysis risk consultancy, Reuters reports that the shooter was likely" ethnically Norwegian," which "could indicate the involvement of a far-right group rather than an Islamist group, though it is also the case that the Labor Party would be a favorable target for Islamist groups due to its role in authorising Norwegian military deployments in Afghanistan."