Anti-Semitic posts on Twitter almost led to the end of Khalif Mitchell's CFL career last year.

And now the commissioner of the league is warning Mitchell that his newly found position back in the league is in jeopardy.

Mitchell, a 31-year-old defensive lineman and former CFL all-star, this week joined the practice roster of the struggling Saskatchewan Roughriders. But when it was announced on Wednesday, a spate of posts Mitchell made on Twitter in mid-August garnered attention. The posts appeared to be anti-Semitic and one declared that the Islamic State was "run by Jews."

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On Thursday, CFL commissioner Jeffrey Orridge said the league would monitor Mitchell's online behaviour, and anything it deems inappropriate "will result in his immediate dismissal."

"Such commentary not only may bring the CFL into disrepute," Orridge said in a statement, "it can promote hate today and dishonour all those who have been subjected to hate over the course of history."

After Saskatchewan signed Mitchell, the Centre for Israel and Jewish Affairs, an Ottawa-based advocacy group, contacted the team and the league, expressing concern about Mitchell "continuing to spread messages of hate against the Jewis h people."

On Twitter Thursday, the group thanked the CFL "for their strong stand against hatred in any form and for taking this matter very seriously."

In Regina, after the Roughriders practised on Thursday, the team's vice-president, general manager and head coach Chris Jones said he had spent an hour on the phone during the morning with the Centre for Israel and Jewish Affairs.

"We're here to play football," Jones told reporters. "If there's anything that's said, then Khalif won't be here."

Mitchell had locked his Twitter account late on Wednesday, on the advice of Jones.

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Asked whether it was a case of being entitled to one's beliefs but keeping them to yourself, Jones said: "Khalif is an outspoken guy and what he's got to understand is if he says something, even if it's his own view, sometimes things are misconstrued, or sometimes they're taken how the people want them to be taken. The less you talk, the better off you are."

Mitchell, after practice, told reporters that there have been "a lot of misinterpretations about myself that were far-fetched."

Team CEO Craig Reynolds said in a statement that he had "spoken with Chris Jones at length about the expectations for all players, including Khalif Mitchell." Reynolds reiterated that "any inappropriate behaviour" from Mitchell would end his tenure with the team.

The CFL, in the commissioner's statement, indicated its social-media policy applies only to current players – but the league publicly announcing it would dismiss a player for violating the policy was an unusual step to take.

Mitchell has a troubled history in the CFL. His first problems came in 2012, for excessive on-field violence and, off the field, a tweet that used a racial slur about Chinese people. Before the 2015 season, he made anti-Semitic posts on Twitter that included a link to Holocaust-denial video. He was fined by the league and the Montreal Alouettes, who then released him.

It is unclear whether the Roughriders considered Mitchell's recent tweets when the team decided to sign him. Jones, on Wednesday, said he had been unaware of the tweets.

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Many Roughriders fans are upset with the Mitchell signing. The Piffles Podcast, hosted by four fans of the team, estimated 98 per cent of fans are unhappy. "I'm embarrassed for the team for stooping that low," one of the hosts said in a new episode.

Josh Hunt, a fan from North Battleford, tweeted: "Can we please just remove him from our team now? That would be great. Signed. #ridernation." Katherine Gelmich of Regina wrote: "In regards to the signing of Khalif Mitchell #notmyRiders."

The Roughriders were the worst team in the CFL last year, at 3-15, and after the season they hired Jones, with a reported salary of $600,000. Jones joined shortly after he had led the Edmonton Eskimos to victory at the 2015 Grey Cup, in his second season there as head coach. This season, however, has been difficult. The Roughriders are 1-8, again the worst in the CFL.

Jones had previously coached Mitchell in Toronto in 2013, as defensive co-ordinator, when Mitchell was last a CFL all-star. The two men recently exchanged text messages, which led to Mitchell's signing. It is unclear when the 6-foot-6 lineman, listed at 315 pounds, might play, given questions about his fitness. The Roughriders play host to the Winnipeg Blue Bombers on Sunday.

The Roughriders are publicly owned and the club's finances are supported by the Saskatchewan's provincial government, with a $3.2-million loan. The loan had originally been $6.2-million but $3-million of the principal was forgiven in 2013 to help the team with the 2013 Grey Cup in Regina.