The story...

On 9/11 there was a reported threat against Air Force Once and/ or President Bush, using the phrase "Angel is next". Angel was a top secret code word known to insiders only, and so Webster Tarpley tells us this threat shows "highly-placed moles within the administration" may have been involved:

According to Bob Woodward’s canonical mainstream account: “At about 10:30 AM
Cheney reached Bush again on Air Force One, which was still on its way toward
Washington. The White House had received a threat saying, ‘Angel is next.’ Since Angel was the codeword for Air Force One, it could mean that terrorists had inside
information.”...

...the “Angel is next” story contained an explosive potential for the longer term, since by pointing toward the existence of highly-placed moles within the administration who had access to top secret code words and procedures, it threatened to explode the official myth of 9/11 which was then taking shape.
Webster Griffin Tarpley
9/11 Synthetic Terror: Made in USA

Our take...

There certainly were many reports of the "Angel is next" message. Tarpley also includes comments from government officials and the mainstream media, repeating the idea that the Angel code word was top secret and that a mole may have been involved:

In an interview with Tony Snow on Fox News Sunday, National Security Advisor
Condoleezza Rice confirmed that the September 11 threat against President Bush’s life included a secret code name.

SNOW: Sept. 11 there was a report that there was a coded message that said, “We’re going to strike Air Force One” that was using specific coded language and made the threat credible. Is that true?

RICE: That is true.

SNOW: So we have a mole somewhere?

RICE: It’s not clear how this coded name was gotten. We’re a very open society and I don’t think it’s any surprise to anyone that leaks happen. So, I don’t know -- it’s possible the code name leaked a long time ago and was just used.

SNOW: How on earth would that happen?

RICE: I don’t know. I don’t know. We’re obviously looking very hard at the situation. But I will tell you that it was plenty of evidence from our point of view to have special measures taken at that moment to make sure the president was safe.

This exchange was reported by Carl Limbacher of NewsMax.com, who added that “U.S. intelligence officials have not ruled out the possibility that a government mole may have given terrorists the top secret code language they used to deliver the threat ‘Air Force One is next’ as the World Trade Center and Pentagon were under attack.”
(NewsMax.com, September 23, 2001)
Webster Griffin Tarpley
9/11 Synthetic Terror: Made in USA

The story faded away over time, though, and the 9/11 Commission Report found the apparent threat was due to a misunderstanding:

1.All times are Eastern Daylight Time. Sometime around 10:30, after the decision had already been made not to return to Washington, a reported threat to "Angel"-the code word for Air Force One-was widely disseminated in the Presidential Emergency Operations Center (PEOC) and aboard Air Force One. Notes from the morning indicate that Vice President Cheney informed President Bush in a phone conversation shortly after 10:30 that an anonymous threat had been phoned into the White House that was viewed as credible.At about the same time, news of the threat was conveyed on the air threat conference call.

The Secret Service's Intelligence Division tracked down the origin of this threat and, during the day, determined that it had originated in a misunderstanding by a watch officer in the White House Situation Room.The director of the White House Situation Room that day disputes this account. But the Intelligence Division had the primary job of running down the story, and we found their witnesses on this point to be credible. During the afternoon of September 11 the leadership of the Secret Service was satisfied that the reported threat to "Angel" was unfounded.

At the White House press briefing on September 12, spokesperson Ari Fleischer described the threat to Air Force One as "real and credible."White House transcript, Press Briefing by Ari Fleischer, Sept. 12, 2001 (online at www.whitehouse.gov/news/releases/2001/09/print/20010912-8.html). Fleischer told us he cited the information in good faith. Indeed, Fleischer had conferred with Vice President Cheney and Karen Hughes before the briefing, and they had decided to let people know about the threat, all of them believing it was true.According to Fleischer, only weeks later did he learn-from press reports-that the threat was unfounded. We have not found any evidence that contradicts his account.Ari Fleischer interview (Apr. 22, 2004); Chuck Green interview (Mar. 10, 2004); Deborah Loewer meeting (Feb. 6, 2004); Ralph Sigler meeting (May 10, 2004); Andrew Card meeting (Mar. 31, 2004); Edward Marinzel interview (Apr. 21, 2004); Secret Service briefing (Jan. 29, 2004).
http://www.9-11commission.gov/report/911Report_Notes.htm

A Wall Street Journal article of 2004 reveals more confusion, and adds an extra detail (our emphasis):

Once airborne, Mr. Bush spoke again on a secure phone with Mr. Cheney, who relayed a new message that changed the president's mind, White House officials later said. The vice president urged Mr. Bush to postpone his return because, Mr. Cheney said, the government had received a specific threat that Air Force One itself had been targeted by terrorists. Mr. Cheney emphasized that the threat included a reference to what he called the secret code word for the presidential jet, "Angel," Mr. Bartlett said in an interview.

In a press conference on Sept. 12, 2001, then-White House spokesman Ari Fleischer said the threat tipped the scales for Mr. Bush. The president reluctantly agreed to remain away from Washington "because the information that we had was real and credible about Air Force One," Mr. Fleischer said.

After a stop in Louisiana, the presidential jet flew to Offutt Air Force Base in Nebraska. Offutt's deep underground bunker gave the president a secure place to hold a video conference with officials in Washington. Shortly after 4 p.m., he decided to return to the capital, arriving at the White House just before 7 p.m.

Although in the days after Sept. 11, Mr. Cheney and other administration officials recounted that a threat had been received against Air Force One, Mr. Bartlett said in a recent interview that there hadn't been any actual threat. Word of a threat had resulted from confusion in the White House bunker, as multiple conversations went on simultaneously, he said. Many of these exchanges, he added, related to rumors that turned out to be false, such as reports of attacks on the president's ranch in Texas and the State Department. As for the Air Force One code name, Mr. Bartlett said, "Somebody was using the word 'angel,' " and "that got interpreted as a threat based on the word 'angel.' " (Former Secret Service officials said the code wasn't an official secret, but a radio shorthand designation that had been made public well before 2001.)

The vice president's office gave an account differing from Mr. Bartlett's, saying it still couldn't rule out that a threat to Air Force One actually had been made.

Days after the attacks, Mr. Cheney had said word of the threat had been passed to him by Secret Service agents. But in interviews, two former senior Secret Service agents on duty that day denied that their agency played any role in receiving or passing on a threat to the presidential jet.

An official in Mr. Cheney's office said in an interview that Mr. Cheney had been mistaken in saying the threat came to him via the Secret Service. The official said that instead, Mr. Cheney had received word of the threat from "a uniformed military person" manning the underground bunker. The official said the vice president and his staff don't know who the individual was. And the official said that he couldn't say definitively whether or not a threat had been made. "I'm not in a position to know the answer to that question," the official in the vice president's office said.
http://www.unknownnews.net/040322911.html

Of course if you believe Cheney was involved in the attacks, and the 9/11 Commission knowingly covered them up, then you’re hardly going to take their word about what happened. And there’s no obvious way to confirm or deny these accounts. Still, there is something relevant we can check here, in the claim that the code "wasn't an official secret, but a radio shorthand designation that had been made public well before 2001". Is this true?

Well, yes: it seems so.

We found one reference of Angel as a code word for Air Force One in a newsgroup "transcript of radio traffic between AF1 and plane carrying cabinet 11-22-63", for instance. This immediately tells us that “Angel” had been in use as a code word for almost 38 years (at least) on 9/11, so plenty of people must have known about it. In addition, the newsgroup post was dated 30th May 2001, and the author claims to have had the transcript since 1994:

The following information relative to Presidential aircraft radio call signs is given for clarification due to interchangeable use. "Air Force One" and "Angel" are used when the President is aboard the aircraft. "Air Force 2600" is used at all other times.
Google Groups Source

We found a second at the Wayback machine's record of 2600.com's Secret Codes page. This lists many code words, and the first version (dated May 10th 1996) includes:

Objects and Events

ANGEL     AIR FORCE ONE
http://web.archive.org/web/19960510165951/http://www.2600.com/secret/more/codes.html

The same information appears in a 2600 newsgroup post dated 12th December 1996.

And if this July 1997 newsgroup post is accurate, then the code word also appears in Jim Keith's 1993 book "Secret and Suppressed: Banned Ideas and Hidden History":

The President and First Lady arrived in Air Force One, codenamed "Angel."
Google Groups Source

Reader Thomas G. (owner of the 11thofSeptember MySpace page) provided the earliest reference, though, showing that the word Angel was used extensively for Air Force One throughout William Manchester's 1967 book, "The Death Of A President". He supplied the following scans by way of support (click on any thumbnail for the full-size version):



This doesn't exactly make the code name common knowledge, but it is confirmation that "Angel" wasn’t top secret, either. If the term was used on 9/11, then it doesn't necessarily imply Government insider knowledge.