Amazon has halted construction of a new U.S. warehouse after seven ropes were found at the site in the past month.
Construction was halted at the Windsor, Connecticut site until security measures were taken.
Amazon said it was “very concerned about the events happening”.
It offered a $ 100,000 prize (£ 70,589) for information on the wires, the first of which was discovered on April 27.
The loop rope is a synonym for hangings, or lynchings, without a sentence, mostly of black people in the United States in the late 19th and early 20th centuries.
Amazon spokeswoman Kelly Nantel said: “We continue to be deeply concerned about the incidents that took place at the construction site in Windsor and ordered it to be closed until necessary security measures can be taken.
“Hatred, racism or discrimination have no place in our society and are certainly not tolerated by Amazon – whether on a site built like this, or on a site we operate.”
She added that the office worked with the town and Windsor police on the matter.
Windsor police said in a statement that the first loop was found hanging on a steel beam inside the building on April 27, before five more ropes “that could be interpreted as hybrids” were found on several different floors two days later.
Construction continued until it was found seventh on Wednesday.
The ropes were hung in unsupervised areas. The statement said hundreds of construction workers employed by a number of different companies worked at the site.
Amazon has increased the proposed reward from $ 50,000 to $ 100,000 in recent days.
However, at a press conference, the president of the Connecticut Chapter of the National Association for the Advancement of Colored People (NAACP), Scottish X Esdaile, said he was disappointed with the response so far.
“We strived to enter the site, to talk to people to make sure they were safe. Still, we have not yet entered the site,” he said.
“It is necessary … that we make sure that those people are safe and that they do not touch.”
State and Connecticut police and Federal Bureau of Investigation (FBI) agents are assisting local police in the investigation.
“The implications of a hanging wire everywhere are unacceptable and will always yield the appropriate investigative response,” FBI special agent David Sanderberg said in a statement to local news site WTNH.