Rose Davis, a spokeswoman for the National Interagency Fire Center in Boise, told The Associated Press that officials there took note of the warning but didn't see a need to act further on it.
The contents of the June 25 memo from the FBI's Denver office were reported Friday by The Arizona Republic. Davis declined to share a copy of the memo and an FBI spokeswoman in Denver didn't immediately return a telephone call.
The Republic reported that the detainee, who was not identified, said the plan involved three or four people setting wildfires using timed devices in Colorado, Montana, Utah and Wyoming that would detonate in forests and grasslands after the operatives had left the country.
The memo noted that investigators couldn't determine whether the detainee was telling the truth.
The newspaper said many forest law enforcement officers it contacted had no idea the warning had been issued.