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 American journalists James Foley and Steven Sotloff.

New York Police Commissioner William Bratton said the department is prepared.

"We will, as always, ramp up intelligence gathering and visibility," Bratton said. That means thousands of officers in specialized teams, bomb-sniffing dogs who can detect not only the scent of a bomb but the vapors of a moving target, undercover officers and teams of police using radioactive detection devices and other high-tech tools.

Intelligence officers around the globe will be reporting in regularly and monitoring events around the world. If something happens in Gaza, it's instantly felt in New York because of the large Jewish and Palestinian populations.

"Things ricochet real quick here," said intelligence chief Thomas Galati.

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 New York, at the Pentagon and near Shanksville, Pennsylvania, in the 2001 attacks, as well as recognizing the six people killed in the 1993 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.

Sandy Kowalchik, 49, of San Jose, California, said she was aware of the security concerns but it didn't change her plan to visit New York this week.

"It seems like authorities in the U.S. and the city are much more aware of anything going on," she said, standing near the reflecting pools at the 9/11 memorial. "Heightened awareness makes people nervous, but it makes you more cognizant of what's around you."

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Associated Press writer Rachelle Blidner contributed to this report.