Chicago: A witness to Saturday's shooting at Donald Trump rally revealed that he saw a man with a rifle on a rooftop a few minutes before shots were fired at the former United States president. 

Greg Smith, speaking to BBC, described how he and his friends, attending Trump's speech, noticed a man crawling atop a nearby building during the event. "He had a rifle," Smith claimed and as soon as he aimed it, they started alerting the police.

"We noticed a man crawling up the roof of the building beside us, 50ft away," he said. "He had a rifle, we could clearly see a rifle…We're pointing at him, the police are down there running around on the ground, we're like 'Hey man, there's a guy on the roof with a rifle'... and the police did not know what was going on."

He tried to alert the officials thinking that the authorities were not able to see the gunman because of the slope of the roof.

"I'm thinking to myself 'Why is Trump still speaking, why have they not pulled him off the stage'... the next thing you know, five shots ring out" he added. 

Following the incident, Trump was swiftly escorted by US Secret Service agents. In his first statement post-attack, Trump disclosed sustaining an ear injury. 

Hinting at the failure of the security, Smith asked "Why is there no Secret Service on all of these roofs here?... This is not a big place…Later the agents crawled up on the roof, they had their guns pointed at him, made sure he was dead.”

Another witness, Joseph, recounted to NBC News how a nearby man was fatally shot during the apparent assassination attempt on the Republican presidential candidate.

"I heard several gunshots. The man beside me suffered a gunshot to the head, was instantly killed (and) fell to the bottom of the bleachers. Another woman looked like she got hit in the forearm or hand," Joseph.

According to Secret Service spokesman Anthony Guglielmi, agents "neutralized the shooter," and the incident is under investigation as an attempted assassination.

Sources report one fatality among the crowd and injuries to two others besides Trump.