Here’s what we know happened on Northwest Flight 327 back on June 29, 2004:

Thirteen Middle Eastern men — 12 from Syria, one a permanent U.S. resident from Lebanon — had one-way tickets for the weekday nonstop from Detroit to Los Angeles. The men, a musical group and their U.S. promoter, took seats all over the plane. Takeoff was delayed when one man with a limp refused to move from an emergency row, pretending he did not understand English.

In flight, one or two of the men walked the aisle, seeming to count passengers. One rushed to the front of the plane in the direction of the cockpit, peeling off at the last moment into the first-class restroom. There he remained for 20 minutes. One of the men went into a lavatory with a large McDonald’s bag and made a thumbs-up sign to another man upon returning. Another made a “slashing motion across his throat, appearing to say “No.” Several of the men spent “excessive time” in the lavatories, and one returned to his seat reeking of toilet bowl chemicals. Several of the men got up when the seat belt sign came on in preparation for landing.

We know all this, not from one of the Flight 327 passengers, though passenger Annie Jacobsen did post an Internet article on the behavior she witnessed and the terror she felt that day. We know all this and more, because of a recent report by the inspector general of the Department of Homeland Security — a report only made public thanks to The Washington Times’ doggedness over two years.

When Jacobsen recounted these suspicious doings a few weeks later and suggested they might be a “dry run” for a terror hit, she became the issue. The government not only denied anything was amiss — nothing here but a bunch of Syrian musicians on their way to a gig, move along now — but air marshals let it be known that she was really more of a concern than the 13 men. And that was the least of it. Some writers mocked Jacobsen as a hysteric, a bigot and a bad writer to boot.

The inspector general report didn’t look into her writing, but it makes clear she was on to something.

It turns out that air marshals were concerned about the men even before they boarded. Six arrived at the gate as a group and proceeded to split and act as if they didn’t know each other. An air marshal said they were sweaty and nervous. Their behavior was, according to one marshal, “unusual.” It also turns out that Jacobsen and other passengers weren’t the first folks on the plane troubled by the men’s behavior. Well before Jacobsen approached the flight attendants about the men, the attendants and marshals were on the case. Law-enforcement officials were waiting for Flight 327 when it landed. They detained all 13 for a short time, releasing them after questioning only two.

Here it gets even more disturbing.

Homeland Security officials failed to report the incident to the Homeland Security Operations Center, the nation’s nexus for information sharing and domestic incident management. This despite earlier FBI warnings that terrorists were planning to use ready-to-build bombs that could be assembled in airplane bathrooms. This despite an FBI warning in April 2004 that terrorists may try to use cultural and sports visas to enter the United States. This despite the fact that the Syrians were traveling on entertainment visas.

Not until two days after the Washington Times reported on the flight and White House terrorism officials asked about the incident did Homeland Security folks enter it in the Operations Center’s logs.

The incident never made it into the government’s National Threat and Incident Database.

In a background check for a visa-extension prior to the flight, eight of the 12 Syrians had “positive hits” for past criminal records or suspicious behavior.

Only when government agencies started investigating the incident did they discover the promoter had been involved in a similar incident with seven other men on a January Frontier Airlines flight from Houston to San Francisco. He was detained in September on a return trip from Turkey, the report says, though the details are redacted.

Was this a dry run or an aborted attack? The Washington Times showed the redacted report to former air marshals. Their conclusion: It could be either. In addition, Flight 327 is not the first such probe.

It hardly matters.

The inspector general’s report reveals a system breakdown — if there was a system to break down. And the fact that the incident was deemed worthy of investigation only after media reports and White House inquiries suggests it might be worse than that.

“I think it now merits an investigation of a cover-up,” terrorism expert Steve Emerson told me.

And how.

Here’s something real Congress should investigate.

And Annie Jacobsen should be the first witness.

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