- Gay nightclub patron says Omar Mateen was a regular visitor
- FBI says Mateen expressed support for bitter rivals, ISIL and Hezbollah
- Mateen's father said his son acted out of anti-gay hatred
- Vigils held across US and in other countries
- 49 people died in worst mass shooting in US history
A number of witnesses have told US media outlets that the perpetrator of the Orlando gay nightclub massacre was a regular visitor at the venue.
Omar Mateen killed 49 people at the site on Sunday in the worst mass shooting in US history before being shot dead by police officers.
The shooter had called 911 during the attack at the Pulse nightclub early on Sunday to express his allegiance to the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group.
Despite Mateen's pledge of support to ISIL, other possible explanations emerged, including questions of whether he was conflicted about his sexuality.
Jim Van Horn, a patron of the Pulse nightclub, told the AP news agency on Monday that Mateen was a regular visitor.
Van Horn said he saw Mateen trying to pick up men at the club and had once spoken to the attacker.
Some of Van Horn's friends then called him away and told him they didn't want him talking to Mateen because "they thought he was a strange person".
Gawker, the Palm Beach Post, and the Orlando Sentinel have also posted articles citing classmates and club patrons suggesting Mateen was gay.
Mateen's ex-wife said he suffered from mental illness and his father suggested he may have acted out of anti-gay hatred.
A confused picture of Mateen's support for armed groups in the Middle East has emerged since the killings.
The 29-year-old expressed support for rival groups that are bitter enemies of one another, such as ISIL, Hezbollah, and al-Qaeda.
The FBI investigated Omar Mateen for 10 months beginning in May 2013 after he was said to have made inflammatory remarks in support of terrorists.
Comey said investigators introduced him to confidential sources, followed him and reviewed some of his communications, but Mateen claimed he made the remarks in anger because co-workers were teasing and discriminating against him because he was Muslim.
US President Obama, meanwhile, said that Mateen appeared to be "self-radicalised", and that there was "no clear evidence that he was directed externally" or that the attack on the nightclub was part of a broader plot.