Orlando gunman's 'changing appearances'

The Orlando attacker drastically altered his appearance on the morning before the massacre, shaving his head and face.

Omar Mateen also stayed up all night researching anti-psychosis medication prior to the attack and showed signs of deteriorating behaviour, Reuters reported.

Three weeks before the attack, a witness said he passed by Mateen at the PGA Village in Port St Lucie, Florida where the assailant had worked as a security guard and noticed agitation in his behaviour.

When asked if he was all right, Mateen said he had been up all night researching psychiatric medication.

"He’d been real worried about whether or not he’d slipped into psychosis," the anonymous acquaintance said. "He wasn’t as friendly. He was obsessed with researching medication online."

"The last month, he looked worried, he looked upset, he looked confused," the acquaintance said. "He didn’t seem himself."

In the early hours of the June 12 attack, the acquaintance said Mateen had not been present to open the gate as usual, however he emerged a few minutes later silent, with a shaven head and face.

When asked if he was okay, the usually polite Mateen responded with "what’s it to you, anyway?"

The 29-year-old killed 49 people and wounded 53 others in a frenzied gun attack at a gay nightclub in Florida. The attack is the deadliest mass shooting in modern US history however the motive has yet to be established.

"It looks increasingly like this may have been the act of a seriously troubled individual whose personal problems dwarfed any last-minute inspiration from radical groups," a senior US official familiar with the investigation, speaking on condition of anonymity told Reuters.