The shooting at the Orlando gay nightclub called Pulse on Sunday that killed 49 people was the worst ever terror attack since 9/11. The gunman, Omar Mateen, was shot and killed by the police when he tried to escape, but as Democratic presidential candidate Hillary Clinton said, “the virus that poisoned his mind remains very strong.”

Mateen even called the 911 to pledge allegiance to the Islamic State during the shooting. Later, the Islamic State claimed responsibility for the attack, but it is still not clear as to how closely connected Mateen was to the group.

Investigations on the massacre are still on. However, the following are five clues from the findings so far which hint at pre-planning of the attack.

  1. Mateen broke into the nightclub and opened fire on the crowd at around 3:00 a.m. and the siege continued until the police managed to enter the club. Time reported that the mass shooting was after the message by the Islamic State in which the group’s spokesman urged jihadists around the world to carry out attacks in their home states during the holy month of Ramadan. In the audio clip that was released on May 22, ISIS spokesman Abu Muhammad al-Adnani especially appealed to “the fighters and supporters of the caliphate in Europe and America.”
  2. Mateen came well prepared with an AR-15 assault rifle, a handgun, and another “device.” “It appears he was organized and well-prepared,” said Orlando Police Chief John Mina. According to FBI agent Ron Hopper, Mateen had legally purchased at least two firearms in the last week. However, it is not yet known whether he used those during the attacks.
  3. Mateen mentioned Abu Bakr al-Baghdadi, the ISIS leader, Boston Marathon bombers, and a man from Florida who died as a suicide bomber in Syria in his calls, FBI Director James Comey said.
  4. Mateen was interviewed twice in 2013 by the FBI. It was later found that he knew the 2014 American suicide bomber from Florida named Moner Mohammad Abu-Salha. But the investigations were closed, according to Hopper, as they were determined that the contact was minimal, the NBC News reported.
  5. According to Mateen’s father, his son nurtured strong hatred towards homosexual people and that he carried out the attack out of homophobia. “This had nothing to do with religion,” the Vox quoted him as saying.