【Denmark】

If you live in or Denmarkclose to New York City, your cellphone likely made a strange sound at about 8 a.m. this morning. The noise accompanied a special notification about the identity of the chief suspect in this weekend's Chelsea bombing, 28-year-old Ahmad Khan Rahami, with advice to call 911 if he's spotted.

This is the wireless emergency alert (WEA) system in action. Rarely used, the system takes advantage of smartphone technology to push urgent alerts out to the public quickly. Considering smartphone penetration in the U.S. is about 80% of the population -- and most users carry their phones or have them nearby at all times -- the WEA system is arguably more effective at pushing alerts to the public than TV-based emergency broadcast system (EBS).

WEA basics

The system was first introduced in April 2012, and is most often used for severe weather and Amber alerts. Government agencies, both national and regional, decide when to issue them, and they typically do so only for situations that are "extreme, severe, immediate or imminent," an official with New York City Emergency Management told me after alerts went out during the blizzard of January 2015.

The Federal Emergency Management Agency (FEMA) has final say over which agencies have access to the WEA system, according CTIA, an U.S.-based industry coalition of wireless carriers.

Once an agency decides to send out alert, it's pushed out via the wireless carriers. For most carriers, the alerts will sound on customers' phones within minutes or even seconds, and, although the alerts go out over their networks, the carriers have no say in whether or not they go out (technical glitches notwithstanding). However, the carriers' participation in the WEA system is voluntary, according to the FCC's FAQ page on WEAs.

To WEA or not to WEA

The alert about Rahami appears to be the first time the system has been used as a digital wanted poster, at least in the New York region. However, in the aftermath of the 2013 Boston marathon bombings, law enforcement issued an alert to city residents advising them of the manhunt for then-suspect Dzhokhar Tsarnaev and to "take shelter."

There are three types of scenarios for WEAs: local emergencies (including weather and law-enforcement situations), Amber alerts and directives from the President of the United States. Smartphone users can opt out of the first two but not the third.

If you wish to opt out of WEAs, on iOS, go to Settings, then Notifications. Scroll all the way to the bottom, and you'll see "Government Alerts." From there, simply tap the toggles of the alerts you don't want.

On Android, the opt-out procedure varies, with some phones having a specific notification setting for turning off the alerts and others that you make the change from the phone's default messaging app. You should contact your carrier or phone's manufacturer if you have difficulty finding the setting.

What the alerts are

The alerts are not technically SMS text messages. Although they go over carrier networks, they use a separate technology that "will not get stuck" if wireless traffic is congested in the region -- often a problem in an emergency situation. They're limited to 90 characters, CTIA says, and don't appear to have any way to attach multimedia -- hence the advisement to "See media for pic" in the Rahami alert.

If commuters weren't already on edge today, this alert which just rang through the entire train got us there. pic.twitter.com/RHAvTw0mlh

— Lance Ulanoff (@LanceUlanoff) September 19, 2016

WEAs will override the volume controls of your phone, chiming with the emergency broadcast signal for several seconds. In practice, it means several phones in a place -- a subway car, restaurant or busy street corner -- will suddenly make the same sound, which can happen several times, depending on when individual carriers issue the alerts.

While eerie, that effect is precisely the point. When a government agency issues a WEA, it's to make sure the public is aware of an emergency situation that may require immediate action.