On Aug. 18, 1964, officials in the Denver Police Department alerted the FBI to a threatening letter brought to them by a local promoter.

Verne Byers, a musician and Denver nightclub owner, had booked England’s most famous rock ‘n’ roll band to perform at the famed Red Rocks Amphitheatre in Morrison the following week.

The Beatles were coming to Colorado.

But eight days before the show, a piece of paper arrived in Byers’ hands. The letter, which had arrived inside an envelope postmarked in Greeley, was comprised of cut-out letters from a magazine taped onto plain white paper.

“If you know what’s good for you, cancel Denver engagement,” the letter read. “I’ll be in the audience and I’m going to throw a hand grenade instead of jelly babies,” referring to the British candy fans famously hurled at the rock stars during the concerts.

The letter’s signature: “Beatle Hater.”

This little-known account is tucked away in the FBI’s once-secret Beatles’ files, a collection of documents that detail the American government’s surveillance of the Fab Four in the 1960s and ’70s — particularly John Lennon’s anti-war activity. The files only became public after a historian fought for their disclosure before the U.S. Supreme Court.

The Colorado letter made its way to the FBI’s director, J. Edgar Hoover, and marked the most serious threat on the Beatles’ legendary 1964 North American tour, a marathon 32 performances in 33 days that transformed the concert business in the United States and helped catapult the British band into another stratosphere of global fame.

Nearly 60 years after John, Paul, Ringo and George descended on the Centennial State, the Beatles are back in the news, and on the Billboard charts, with the release of a new — and final — song.

This is the story of the Beatles’ only trip to Colorado — and the threat that could have ruined it all.

Mayhem at the Brown Palace

By early 1964, the Beatles had achieved royalty status in their native England. But Americans had not yet been fully introduced to the famed foursome.

That all changed when the lads from Liverpool appeared on “The Ed Sullivan Show” in New York on Feb. 9 of that year. Some 73 million people tuned in to the hour-long broadcast as the group performed “All My Loving,” “She Loves You,” “I Want to Hold Your Hand” and other hits.

The show brought Beatlemania to American living rooms — but the 1964 North American tour brought the rock stars directly to their fans.

From the moment the band members landed in San Francisco in August to begin their first U.S. tour, they encountered pure, unadulterated bedlam.

Seconds into their first show there, fans rushed the stage, injuring 19 people. The concert twice had to stop after attendees chucked jelly beans at the performers. The Beatles, after the show, were forced to escape in an ambulance when the crowd mobbed their limousine. Girls at the Vancouver concert were so overwhelmed they vomited.

On Aug. 26, the Beatles arrived in Denver for the sixth stop on the tour. An estimated 10,000 people waited at Stapleton Airport to greet the British sensations, with thousands more lining the streets as a motorcade whisked the band to the Brown Palace Hotel downtown.

The commotion at the hotel grew out of control, earning six crowd members and one police officer a trip to the emergency room, Colorado Public Radio reported in a look back at the Beatles’ ’64 visit.

The band stayed in suite 840 and ordered room service: Five grilled cheese sandwiches, fries and soda, author Chuck Gunderson wrote in his book “Some Fun Tonight!,” which chronicled the Beatles’ three North American tours in the mid-’60s.

The Red Rocks show, though, almost didn’t happen.

Byers nearly fell off his chair when the Beatles’ manager said it would cost $20,000 to book the band, Gunderson wrote — an amount seven times what other musicians charged.

The owner of the Kansas City Athletics baseball team, Charlie Finley, even offered Byers a significant sum to buy the concert and move it there, the author found.

Byers refused.

The Beatles were offered two options for their Denver show: the 22,000-person University of Denver Stadium — which has since been demolished — or the 9,500-seat Red Rocks Amphitheatre in nearby Morrison.

Brian Epstein, the Beatles’ manager, opted for the smaller location because “he felt it was too great a risk to try and fill a 22,000-seat venue in a lower population market,” Gunderson wrote.

Inside the FBI investigation

At least one person, however, wasn’t pleased to see Beatlemania come to Colorado.

The FBI, upon learning of the hand-grenade threat, immediately sent the envelope and letter to a laboratory to scan for fingerprints, according to 25 pages of internal documents posted to the agency’s website.

They came back with no hits.

The threat, according to one memo, fell within the purview of the federal extortion statute.

A U.S. Postal Service inspector in Greeley told authorities he had seen no similar letters, FBI documents show. A Greeley police detective said the same. The FBI even asked Greeley High School whether teachers had seen any similar letters from students over the past year.

A week after the FBI learned of the letter, an unnamed Denver agent wrote that investigators had no information about who might have prepared or mailed in the threat.

Ultimately, the Beatles’ Aug. 26 performance at Red Rocks went off without a hitch, though the FBI files indicate Denver police dispatched 200 officers to handle the crowd. The Jefferson County Sheriff’s Office sent an additional 50 people to assist.

On Sept. 18, one month after Denver authorities went to the FBI, the bureau wrote that it had not developed any suspects or subjects and “no further investigation is being conducted.” The case would be closed.

Denver wasn’t the only stop on the tour that included safety threats against the band. In other cities, police sent bomb-sniffing dogs into arenas before concerts, Gunderson said in an interview. Other threats drew extra law enforcement scrutiny.

“They drew so much attention wherever they went,” Gunderson said. “It was very easy for people to get in there and make claims or threats.”

Still, the Denver show represented the most credible threat the Beatles faced on the tour, he said.

George Martin, the Beatles’ producer, later recalled climbing up to the top of the Morrison amphitheater and looking down at the band.

“The amphitheater is such that you could have a sniper on the hill who could pick off any of the fellows at any time — no problem,” Martin said in a 2000 interview. “I was very aware of this, and so was Brian (Epstein), and so were the boys.”

The FBI started keeping files on the Beatles as soon as they landed in San Francisco at the start of the tour, Gunderson said. Agency documents note the tour could be used “as (a) possible vehicle through which riot condition might be brought by an outside organization.”

Later, in the 1970s, the Richard Nixon administration surveilled Lennon, worried that the rock star might combine music with politics to urge young people against voting for the Republican in the 1972 election.

“Don’t be rowdies”

It’s unclear whether the public at the time had any knowledge of the anonymous threat against the band.

News accounts from 1964 seemed more concerned about Denver teens making fools of themselves.

“You have the opportunity of attracting worldwide attention today!” a Rocky Mountain News column printed the morning of the concert read. “Don’t be rowdies. Don’t throw things. Don’t try to smuggle beer or liquor into Red Rocks Theatre. Don’t kick and elbow. Gird on the self-discipline that is the mark of a true American citizen.”

Tickets for the show cost $6.60 — double the usual rate. Despite reporting that suggested there were empty seats because the concert didn’t sell out, Gunderson said lax enforcement from ushers meant legions of fans were able to sneak into the venue without paying.

Another misnomer: There’s no evidence members of the band took hits from oxygen canisters on the stage due to the high altitude, he said.

McCartney, in a later interview, admitted it was hard to breathe during the Red Rocks show.

“It was an interesting experience, physically,” he said.

Joan Baez, the famed singer-songwriter in town for a gig, visited the band backstage.

The Beatles played that night for only 35 minutes. They never came back to Colorado.