The Islamic State has taken responsibility for the savage mass shooting at an Orlando gay club late Saturday that left 50 people dead and 53 others injured. It was the deadliest mass shooting in American history. "The attack that targeted a nightclub for homosexuals in Orlando, Florida and that left more than 100 dead and wounded was carried out by an Islamic State fighter," the terror group said in a statement.

ISIS took credit for the attack via Amaq Agency, a news agency affiliated with the terror group.

Sen. Bill Nelson (D-FL) confirmed that ISIS has released the statement on Sunday afternoon when speaking with reporters.

“In this case, if we’re getting those kind of statements from the news agency of the Islamic State, we’ll have to see what those connections are,” he stated.

29-year-old shooter Omar Mateen reportedly pledged allegiance to ISIS and its leader Abu Bakr al Baghdadi in a phone call he made to police during the shooting.

Additionally, the shooter had been investigated by the FBI in the past. “The senior law enforcement source reports that Mateen became a person of interest in 2013 and again in 2014,” reports The Daily Beast. “The Federal Bureau of Investigation at one point opened an investigation into Mateen but subsequently closed the case when it produced nothing that appeared to warrant further investigation.”

The same source told The Daily Beast that Mateen was a “known quantity," adding that "[Mateen's] been on the radar before.”

In an audio message posted in May, ISIS spokesman Abu Mohammed al-Adnani, urged the group’s followers to carry out terrorist attacks during the Islamic holy month of Ramadan:

Ramadan, the month of conquest and jihad. Get prepared, be ready...to make it a month of calamity everywhere for the non-believers...especially for the fighters and supporters of the caliphate in Europe and America.

Whether Mateen had direct or indirect links to ISIS is yet to be determined. Federal officials also haven’t determined the validity of ISIS’ statement taking credit for the attacks. The Islamic State state hasn’t provided any clear evidence proving its involvement, but the circumstantial evidence is mounting.