Authorities across the US are investigating after reports of text messages sent to black Americans with references to “slave catchers”, plantations and picking cotton.

In a statement the FBI said it is “aware of the offensive and racist text messages sent to individuals around the country and is in contact with the Justice Department and other federal authorities on the matter.”

The source of the messages and the total number sent are unclear, however, there are reports that they were received in at least 15 states and Washington DC.

Some of the messages mentioned the Trump campaign – which strongly denied any connection.

Steven Cheung, a campaign spokesman, said: “The campaign has absolutely nothing to do with these text messages.”

According to examples posted online and cited in news reports, the wording of the messages varied but generally instructed recipients to report to a “plantation” or wait to be picked up in a van, and referred to “slave” labour.

The messages appear to have started on Wednesday, the day after election day. Among the recipients were college students and children.

In a statement Derrick Johnson, head of the civil rights group NAACP, said: “These actions are not normal.”

“These messages represent an alarming increase in vile and abhorrent rhetoric from racist groups across the country, who now feel emboldened to spread hate and stoke the flames of fear that many of us are feeling after Tuesday’s election results,” Johnson said.

Jessica Rosenworcel, chairwoman of the Federal Communications Commission, which is also investigating the messages, said: “These messages are unacceptable. We take this type of targeting very seriously.”

The messages were reportedly received across southern states, New York, Pennsylvania, Maryland, Ohio, California, Washington DC and others, US media say.

One recipient, Hailey Welch, told a University of Alabama student newspaper that several students on the campus had also received the messages.

“At first I thought it was a joke, but everyone else was getting them. People were texting, posting on their stories, saying they got them,” Ms Welch told The Crimson White. “I was just stressed out, and I was scared because I didn’t know what was happening.”

In several states, top law enforcement officials said they were aware of the messages and encouraged residents to report them to the authorities if they received them.

The office of Nevada’s attorney general said it was working to “probe into the source of what appear to be robotext messages”.

The office of Louisiana’s attorney general said it had discovered that some of the messages could be traced back to a VPN in Poland, but that “no original source” had been found so far.