Blackhawks and A/MH-6 Little Bird helicopters used by special forces circling low through downtown Chicago skies made for a scary scene along the Chicago River, as they rattled windows flying among the city’s skyscrapers.

The training exercise comes as the city found out some of the details of what the Secret Service wants for security at next month’s summit at McCormick Place.

FOX Chicago News has their terror prevention shopping list, courtesy of the Chicago Sun-Times:

Portable high security barriers
Crowd control barriers
10-12 portable light towers
Sandbags, port-a-potties
Mobile offices
5-10 golf carts

The Chicago Tribune reports that the most traffic for NATO dignitaries will be between McCormick place and downtown hotels in the Loop, River North and the Gold Coast.

Fifty heads of state will attend, with delegates, and will be moving multiple times each day.

FOX Chicago News viewers called in, describing men with automatic weapons, hanging out of helicopters.

The city’s Office of Emergency Management and Communications on Monday announced that Chicago is the site of a military training exercise this week involving personnel and at least three helicopters.

But the bland press release from City Hall did nothing to prepare folks for the jarring scene that unfolded Monday evening.

“It was frightening,” Jessica Hill said. “I was definitely alarmed.”

At least three Blackhawk helicopters flying at time in close order formation – at other times separating fairly widely – zoomed around office and residential towers for several hours.

“I was pretty amazed,” Felise Llano said, “because they were definitely military.”

Witnesses described three to four very heavily armed men in each chopper. Several said some of the men appeared to be tethered and were at times literally hanging out of the choppers, and looked ready to jump.

Calling it routine training, city officials said in a statement such exercises are conducted by military personnel in various cities around the country. They say the exercises are designed to improve the military’s ability to operate in urban environments.

Emergency management officials said the training sites have been selected to minimize the impact on residents’ daily routines.

The timing of the drills led some to wonder if they’re in preparation for next month’s NATO summit.

But officials said that the summit and the drills are unrelated.

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