Omar Mateen pledged loyalty to the Islamic State, but also expressed solidarity with the Tsarnaev brothers and support for al-Qaida and Hezbollah.
The suspect in the Orlando attack was identified as Omar Mateen of Port St. Lucie, Florida.
Concerns about nuclear terrorism rose after Belgian media reported that suicide bombers who killed 32 people in Brussels on March 22 originally looked into attacking a nuclear installation before police raids that netted a number of suspected associates forced them to switch targets.
European and Iraqi intelligence officials and a French lawmaker who follows the jihadi networks described camps in Syria, Iraq and possibly the former Soviet bloc where attackers are trained to attack the West.
Belgian prosecutor Federal Prosecutor Frederic Van Leeuw identified two of the Brussels attackers as brothers: Ibrahim El Bakraoui, a suicide bomber at the airport, and Khalid El Bakraoui, who targeted the subway.
Belgian media said police were hunting one attacker who had survived.
There was no credible claim of responsibility eight hours after the first blast; but the coordinated assault, four days after Brussels police captured the prime surviving suspect in the Islamic State attacks on Paris, turned immediate attention to local Islamists.
"We do know it would better if we can reach a political solution but we are prepared ..., if that's not possible, to have a military solution to this operation and taking out Daesh," Biden said at a news conference after a meeting with Prime Minister Ahmet Davutoglu.
A suicide bomber affiliated with the Islamic State group detonated a bomb near the Blue Mosque killing at least 10 people - nine of them German tourists - and wounding 15 others.
El alcalde de esa ciudad de Bélgica señaló que tuvieron que anular las celebraciones por razones de seguridad