NEW YORK — Air strikes in Iraq, ongoing unrest in Syria and the beheadings of two American journalists are casting a long shadow over the 13th anniversary of the Sept. 11 attacks.

While there is no specific threat against New York ahead of the Thursday commemoration, the rising power of disparate militant groups around the world presents the most complex terrorism danger since the twin towers were destroyed, New York intelligence officials said this week.

“It is layer upon layer upon layer – not all coming from the same place or ideology,” said John Miller, the New York Police Department’s deputy commissioner of intelligence and counterterrorism.

That differs from five years ago, when the risk was chiefly from al-Qaida, Miller said. Now, he said, the threat is also coming from the well-funded, highly sophisticated “mass marketing of terrorism” – affiliate groups, foreign fighters, uprising militants and the idea of “al-Qaida-ism.”

“When you look at the level of sophistication, the amount of slickness applied to their video production, the amount of thought that goes into creating a narrative,” he said, “They’re doing the same kind of thing as we’ve seen in commercial publishing or in the ad industry.”

New York remains the top target, and that makes preparing for big events, including the U.S. Open tennis tournament, the United Nations General Assembly and the Sept. 11 commemoration, that much more critical, officials said. Plus, President Barack Obama plans this week to outline an expanded U.S. campaign against militants in Iraq and Syria following the beheadings of two American journalists.

“We will, as always, ramp up intelligence gathering and visibility,” New York Police Commissioner William Bratton said.

That means thousands of officers in specialized teams, bomb-sniffing dogs, undercover officers and teams of police using radioactive detection devices and other high-tech tools.

The private anniversary ceremony will be held on the National September 11 Memorial & Museum plaza on Thursday morning. The tribute has centered on reading the names of the nearly 3,000 people killed in the 2001 attacks, as well as recognizing the six people killed in the 1993 World Trade Center bombing.

But for the first time, the memorial plaza will be open to the public this year from 6 p.m. to midnight.

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