A look at what the FBI is investigating in relation to the Orlando nightclub massacre

The FBI has opened a criminal investigation into the shooting at Pulse nightclub in Orlando that left 50 people dead, the Justice Department announced on Monday.

The Orlando police chief said in a statement that the bureau is investigating the possible links between the shooter and radical Islam, and he said there is a possibility the suspect was inspired by Islamic State militants.

Authorities have said the shooter, Omar Mateen, was a US citizen and a citizen of Afghanistan.

He had been on a US passport for nearly two years.

A second shooter, who police believe was killed by authorities, was also believed to be part of the attack, but authorities have not said whether he was an American citizen.

Federal authorities have previously said that Mateen had been in the US for about three years, but that they had not previously learned he was not a naturalized citizen.

He was a resident of the Orlando suburb of Miramar for about 10 years, according to the Justice Dept.

The shooting took place at a gay nightclub in the heart of Orlando, a popular tourist destination.

It left 49 people dead and hundreds injured.

Mateen, who was killed in a shootout with police, had been living in the United States for at least two years, a person familiar with the matter told ABC News.

The official said Mateen did not have any prior convictions.

Maddock’s sister, Zubeidat, who said she was not involved in the attack but spoke to the Associated Press news agency by phone, said the gunman had been diagnosed with bipolar disorder and depression.

The FBI is also investigating the possibility Mateen was inspired in part by the Islamic State militant group.

The White House said President Donald Trump called the attack a “horrific act of terror” and said the United Kingdom has also sent a message to the Islamic world that “America will stand with you.”

Trump also tweeted that the attack was “just another attack by radical Islamic terrorists who seek to destroy our way of life.”