Philadelphia police are investigating a threat of violence against the African-American Museum during this month's Democratic National Convention.
The museum, at 7th and Arch streets in Center City, received an anonymous letter Thursday morning containing the threat, police and museum staff said.
Law enforcement sources say the letter mentioned a group, possibly white supremacists, would carry out the attack.
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A police spokesperson would only confirm that the letter was received and that detectives were investigating.
Museum president Patricia Wilson Aden said the museum is withholding comment until more information is uncovered by the police investigation.
The museum remains open to the public.