Image copyright Twitter Image caption Beer sponsors said they would no longer make a can with Mr King's likeness

An Iowa newspaper journalist who exposed racist tweets by a sports fan at the centre of a viral charity fundraiser has himself been fired after his own offensive posts resurfaced.

Des Moines Register reporter Aaron Calvin has lost his job after his profile of Carson King triggered fury.

The newspaper has been forced to hire extra security due to threats.

The story has provoked a debate on US "cancel culture", where public figures are hounded for causing offence.

Carson King, an Iowa State University football fan, found unexpected fame on 14 September after his hand-drawn cardboard sign requesting donations for his "Busch Light Supply" was featured in the background of an ESPN broadcast.

After initially receiving around $600 from amused sports fans, the 24-year-old announced the money would be donated to the local children's hospital, causing donations to skyrocket.

Venmo, the money-transferring website that Mr King had cited on his poster, and Anheuser-Busch, the brewer of Busch Light lager, offered bumper donations, in addition to a multitude of Iowa businesses, totalling $1.8m (£1.5m).

The state governor proclaimed 28 September "Carson King Day", saying his "volunteerism and selflessness defines Iowans by nature", and "demonstrate how Iowa Nice isn't just a slogan, but our way of life".

It was around then that Des Moines Register reporter Aaron Calvin began writing a feature on Mr King, which was published on 24 September.

It said that while in high school eight years earlier, Mr King had tweeted two racist jokes about black people.

Image copyright CBS/ESPN

Just before the Des Moines Register story came out, Mr King, who works as a casino security guard, convened a press conference to apologise.

"I am so embarrassed and stunned to reflect on what I thought was funny when I was 16-years-old," he said of the old tweets.

He emphasised that the state's largest newspaper "has been nothing but kind in all of their coverage, and I appreciate the reporter pointing out the post to me".

"I want to sincerely apologise," Mr King said. "Thankfully, high school kids grow up and hopefully become responsible and caring adults."

Critics of the newspaper condemned its decision to dredge up Mr King's offensive posts, which were included near the bottom of the article.

Busch Light began distancing itself from Mr King, but said it would still honour its $350,000 commitment to the University of Iowa Stead Family Children's Hospital.

The brewing giant said, however, it no longer planned to send Mr King a year's supply of beer with his face printed on the limited-edition cans.

Online backlash against the backlash built even before the Des Moines Register article was published, and Twitter sleuths began uncovering Mr Calvin's own statements that mocked same-sex marriage, domestic abuse and included a racial slur.

In a contrite tweet, he wrote: "Hey just wanted to say that I have deleted previous tweets that have been inappropriate or insensitive.

"I apologise for not holding myself to the same high standards as the Register holds others."

His apologetic tweet was deleted, too.

Late on Thursday night, the executive editor of the Register announced that Mr Calvin had been fired after his online comments were brought to the paper's attention.

Editor Carol Hunter's letter to readers says: "We hear you: You're angry, you're disappointed and you want us to understand that."

She wrote that the paper was re-examining employees' social media accounts as well as policies for "backgrounding individuals in stories, with particular attention to acts committed by juveniles and to the newsworthiness of that information years later".

As of Friday, more than 166,000 people have signed a petition calling for the Register to issue a front-page apology to Mr King "for trying to berate and shame a young man whose goal was to do something truly amazing".

In a Twitter post on Thursday, Mr King thanked his supporters, adding that "a special thanks to the kids and families of Stead Family Children's Hospital who have shared their stories. Your stories remind us why this is all worth it".

"This is not about me, though. This is about all of us. I am just overwhelmed that a sign that started as a joke will end up making such a meaningful impact for years to come."

"Our society can be so divisive at times," his letter continued.

"But these two weeks have shown we have the power to come together to make a difference. I hope this can be an inspiration for all of us going forward."