CHICAGO (CBS) — Military helicopters buzzed the Chicago skies Monday night, and they had many people wondering what was going on.

WBBM Newsradio’s Bernie Tafoya reports a group of helicopters was part of routine military urban training taking part in cities across the U.S. Those exercises run through Thursday in the Chicago area.

READ MORE: Northwestern University Bans All Social Activities At Campus Fraternities Until At Least Mid-October After Reports Of Drugging

Monday night, plenty of people took to Twitter about what they saw or heard.

One person tweeted: “It’s been an all-night invasion.”

READ MORE: Suspect Tries To Break Into ATM In Portage Park Using Blow Torch

A woman said, “Blackhawk helicopters flying over my condo in the Loop, and all news stations are covering the royal birth, what’s up?”

Some people thought it meant President Barack Obama was visiting town.

MORE NEWS: 1 Dead, 3 Injured After Shooting In Humboldt Park

The city’s Office of Emergency Management and Communications said training sites were carefully selected to minimize impact on citizens.