The sniper fired more than 70 rounds from his rooftop perch, randomly picking off victims below at the July Fourth celebration in the affluent Chicago suburb.

The man held by police in connection with the mass shooting at a July Fourth parade allegedly planned the attack for weeks and dressed like a woman to avoid detection, authorities said on Tuesday.

Robert “Bobby” E. Crimo III,, who was arrested by police hours after the rampage, scaled a fire escape ladder to make himself a sniper's nest to fire on paradegoers below on Monday, authorities said.

“But we do believe Crimo preplanned this attack for several weeks,” Chris Covelli, spokesman for the Lake County Major Crime Task Force, told BBC reporters on Tuesday.

Covelli also revealed that Crimo dressed in women's clothes during the rampage, which killed six people and injured dozens more during the celebration in the wealthy Chicago suburb. 

Authorities didn’t immediately detail the attire Crimo allegedly wore on Monday, or if he wore a wig.

“During the attack, Crimo was dressed in women’s clothing and investigators do believe he did this to conceal his facial tattoos and his identity and help him during the escape,” Covelli said.

That plan seemed to work initially, as Crimo allegedly walked undetected to his mother's home, borrowed her car and drove out of town.

"Following the attack, Crimo exited the roof, dropped his rifle and he blended in with the crowd and he escaped,” Covelli said.

“He blended right in with everybody else as they were running around almost as he was an innocent spectator as well.” 

Investigators managed to piece together Crimo's movements based largely on video footage taken on Monday in downtown Highland Park.

"He was seen on video camera in the women's clothes. Video camera played a tremendous role in how we were able to identify him both leading initially and as he left," Covelli said.

The sniper fired more than 70 rounds from his rooftop perch, randomly picking off victims below, according to officials.

Covelli said there was no immediate evidence that the shooter was aiming for victims based on race or religion.