El Paso County Sheriff's Office said it received several phone calls Sunday from people who found the invitations taped to their mailboxes, The (Colorado Springs) Gazette reported.
The invitations included literature targeting racial groups and a phone number to a voice mail that asserted white supremacy.
"We contacted the postal inspector, who indicated this was a violation of federal law," El Paso County Sheriff's Lt. Jeff Kramer said. "The Sheriff's Office will play an assistant role to the investigation."
The Postal Service is investigating.