The bodies of the Orlando, Florida shooting victims were removed from Pulse nightclub overnight, but investigators are working Monday to determine what motivated the killer responsible for the worst mass shooting in US history.

Omar Mateen was known by the FBI to be sympathetic to terrorism and had been previously investigated.

He was possibly planning to carry out the massacre at the Orlando gay club for a long time.

In recent weeks, he had attempted to buy military-grade body armor from a local store, according to a US official.

The store refused to sell him what is called Level III body armor.

Level III body armor offers more protection than the body armor police typically issue to officers.

Mateen had also legally purchased a Glock pistol and a long gun, according to the Buereau of Alcohol, Tobacco, Firearms and Explosives.

It’s not known if these weapons were used in the attack.

The FBI and local law enforcement have been examining Mateen’s employment history.

Since 2007, he worked as a security officer at G4S Secure Solutions, one of the world’s largest private security companies.

He was assigned to the St. Lucie County Courthouse. His company knew the FBI had interviewed him, but Mateen passed the company’s background check.

He was interviewed by the FBI in 2013 and 2014 after expressing sympathy for a suicide bomber, according to Assistant Special Agent Ronald Hooper.

Hooper says the interviews were inconclusive – meaning there was no reason to keep them going.