Authorities are investigating a mass shooting threat against Walmart stores in Kansas City, Missouri, Kansas City Police Detective Robert Jorgenson told Business Insider in an interview Saturday.

The threat was posted this week to Reddit by someone claiming to have two AR-15 semi-automatic weapons and 22 pipe bombs, according to a copy of the threat viewed by Business Insider.

The Kansas City Police Department has assigned off-duty officers to local Walmart stores as the investigation continues.

Walmart told Business Insider it is aware of the threat and has added additional security.

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Authorities are investigating a social media post threatening the "biggest mass shooting in modern American history" at a Walmart store in Kansas City, according to Kansas City Police Detective Robert Jorgenson.

"The investigation is ongoing," Jorgenson told Business Insider on Saturday.

The threat was posted this week to Reddit by someone claiming to have two AR-15 semi-automatic weapons and 22 pipe bombs, according to a copy of the threat viewed by Business Insider. The Reddit post suggested the shooting could take place at a Walmart in Kansas City or Independence, Missouri.

The post's author said they are from Independence. They claimed to be planning an imminent attack with a "friend" and said a "large note" on their phones would reveal information about their motives.

Read more: Walmart workers grapple with heightened fears in the aftermath of deadly shootings

The FBI, police, and Walmart are all investigating the threat

Multiple law enforcement agencies, including the FBI, are investigating the threat, according to the Kansas City Police Department.

Jorgenson said he could not provide information on whether a suspect has been identified or any other details while the investigation is ongoing.

The Kansas City Police Department has assigned off-duty officers to local Walmart stores for added security as the investigation continues, he said.

A Walmart spokeswoman told Business Insider the company was aware of the post and has been "in close contact" with stores.

"We take threats like this seriously and have added additional security as appropriate," the spokeswoman said. "We will continue to help federal and local authorities with their investigation to determine the source and any credibility of a threat, which has not been substantiated."

A police officer stands next to a police cordon after a mass shooting at a Walmart in El Paso Reuters

The threat comes after shootings at two Walmart stores last week that killed at least 24 people in El Paso, Texas, and Southaven, Mississippi.

In a post to Facebook on Friday evening, the Kansas City Police Department said it was getting "a ton" of messages regarding the social media threat.

"We are getting a ton of messages right now regarding a threat of violence at a local Walmart that was posted on Reddit and elsewhere," the Facebook post said. "We became aware of this threat yesterday and have been working with our federal partners to address it. Thank you for your vigilance."

The department posted an update on Saturday that said: "As of this morning we are working closely with the FBI and other law enforcement partners to investigate this threat and vet the information that is being developed. Thank you to everyone that has been keeping an eye out and alerting us to this concerning post."