Chilling warning signs of gunman who attempted to storm the White House Correspondents' Dinner were revealed on Sunday. 

Cole Thomas Allen, 31, will be arraigned on Monday after he allegedly shot a Secret Service agent while trying to attack the annual gathering, with President Trump and several other administration members in attendance.

A law enforcement official briefed on the investigation told The Daily Mail that interviews with Allen's siblings revealed the family may have missed something.

‘It sounds like they certainly had some concerns about his behavior. So we’re certainly looking at what, at what point, did we miss a warning sign in looking at some of his rhetoric,' the official said.  

Officials received Allen's manifesto from his brother when he reached out to the New London, Connecticut Police Department in the wake of the shooting.  

'The problem is we’re in such a charged climate. I mean, there’s a lot of commentary on social media just about the state of affairs,' the source told The Daily Mail. 

'Trying to figure out, when a family member becomes concerned, how to encourage them to report it so things like this don’t happen.'

While they continue investigating Allen's motivations, they know that he had attended a 'No Kings' rally and are concerned about potential copycats.

The official said that they're looking into what can be done differently in the future to identify possible shooters.   

'It’s hard because there is a lot of emotionally charged things going on right now, and people are certainly voicing some of the frustration online and on social media, and sometimes it crosses the line to hateful speech,' the official said. 

'But it’s kind of like using a teaspoon to measure the ocean.'

Both Allen's brother and sister were concerned about his 'increasingly agitated rhetoric' in his writings.  

They noted that he was empathetic toward transgender people and against the Iran War but he was unhappy about several issues and not one specifically.  

Allen, who has no criminal record, may have scheduled sending his manifesto based on how it was written.  

'It appears as though he didn’t know if he was going to be around,' they said. 

They are not yet clear on any past drug use or mental health issues displayed by Allen. 

Allen called himself the 'Friendly Federal Assassin' in a chilling manifesto he wrote before the attack. 

He sent the eerie anti-Trump writing to his family members just 10 minutes before he fired shots at the hotel in Washington, where the ritzy press gala was taking place on Saturday night. 

The harrowing writing was given to the police by a relative, a US official said. 

According to the New York Post, Allen's manifesto read: 'Turning the other cheek is for when you yourself are oppressed. I'm not the person raped in a detention camp. I'm not the fisherman executed without trial.

'I'm not a schoolkid blown up or a child starved or a teenage girl abused by the many criminals in this administration. Turning the other cheek when *someone else* is oppressed is not Christian behavior; it is complicity in the oppressor's crimes.

'In order to minimize casualties, I will also be using buckshot rather than slugs (less penetration through walls). 

'I would still go through most everyone here to get to the targets if it were absolutely necessary (on the basis that most people *chose* to attend a speech by a pedophile, rapist, and traitor, and are thus complicit) but I really hope it doesn't come to that.'

His targets included 'administration officials (not including [FBI Director Kash] Patel): they are targets, prioritized from highest-ranking to lowest.'

Allen wrote, allegedly referencing the president: 'I am no longer willing to permit a pedophile, rapist, and traitor to coat my hands with his crimes.'

Trump spoke out on Sunday morning, following the chaotic night at the dinner.

He told Fox that the shooter 'had a lot of hatred in his heart' and that religion was a reason why the gunman attacked. 

The President said this morning: 'He had a lot of hatred in his heart for quite a while.'

He added that the manifesto was 'a religious thing. It was strongly anti-Christian.'

'He's got some big problems with the rest of his life, but it's very, very bad, very bad situation,' the President continued.

Allen, a teacher from Torrance, California, had a manifesto, and his siblings knew he had firearms and were worried about him.

On Saturday night, chilling surveillance footage captured the moment gun-wielding Allen stormed past security at the White House Correspondents' Dinner.

Allen was seen wearing all black as he sprinted inside the Washington Hilton hotel on Saturday evening with a gun in hand, video posted by President Trump on Truth Social showed.

Just moments before he ran, security guards were seen standing in the hallway, but as soon as he sped by, they immediately reacted and pulled out their firearms.

Police revealed the suspect had a number of weapons on him, including a shotgun, a handgun, and multiple knives.

President Trump shared a stunning image shortly after the shooting, showing the suspect shirtless and lying face down on the carpet.

Law enforcement confirmed he was not struck by gunfire while being stopped, but has since been taken to the hospital for evaluation.

Allen was a guest at the Hilton hotel, where the event was taking place.

Surveillance video shows the suspect sprinting past a security checkpoint and attempting to reach the doors to the ballroom where the president was gathered, along with his most senior cabinet members and thousands of journalists.

Evidence found on Allen's electronic devices and in his writings points towards the theory that he intended to target administration members in attendance at the dinner.

His family members told law enforcement that Allen had sent them some of his disturbing writings before the attack, which prompted one of them to alert police. The writings did not specifically mention the dinner on Saturday.

Another family member told investigators that Allen has made radical statements and that he frequently mentioned plans to do 'something' to fix problems with today's world.

The family added that Allen would regularly visit a shooting range to train with his guns. A senior US official told CBS News that the suspected gunman was part of a group called The Wide Awakes and that he attended a No Kings protest in California.