Many New York-area commuters on Monday were interrupted by this alert on their cellphones: “WANTED: Ahmad Khan Rahami, 28-yr-old male. See media for pic. Call 9-1-1 if seen.”

The alert – sent by the city’s Office of Emergency Management – targeted cellphone users in the area as police ramped up a manhunt for Rahami, who was wanted in connection with the bombings in Manhattan and Seaside Park, New Jersey, over the weekend.

Rahami was caught hours later. But for some cellphone users, lingering questions remain. Is this the first time this system has been used to notify people about a terror suspect? Why didn’t the alert come with a photo of Rahami? How are these messages actually delivered?

To get you up to speed, here’s everything you need to know about the Wireless Emergency Alert system, a key part of the country’s emergency infrastructure.

–Why did I get a Wireless Emergency Alert?

Cellphone users get WEAs when authorities believe they have to notify the public about a weather emergency, a missing child, or an imminent violent threat to people’s safety. The White House is also empowered to send WEAs that you can’t opt out of, even if you’d like to.

–Why didn’t I get a WEA?

WEAs can be geographically targeted down to the nearest cell tower, and up to areas as wide as an entire county. This makes sure that almost everyone who’s within a certain area gets the alert. WEAs are only supported on a device-by-device basis, so if you don’t get these alerts, it’s possible that you’ve either turned off the feature in your settings or your handset doesn’t have the technology. But these days, most devices are WEA-capable, which explains why so many New Yorkers woke up to the emergency alert on Monday.

–So WEAs are just emergency text messages?

Not exactly. The alerts themselves may arrive purely in textual form, but that doesn’t mean they’re text messages like the kind you’d send to your friends and family. In fact, that’s part of the point. During an emergency, the regular cell networks may get overloaded. So the WEA system is designed to transmit messages separately from what the rest of us use.

The technology is known as cell broadcast messaging. It’s different from text messages in a few ways. Text messages require you to know the phone number of each person that you want to reach, essentially creating a point-to-point connection for each message. Cell broadcast doesn’t care about phone numbers at all – it’s virtually guaranteed that whomever is in range of the cell tower will receive the message it’s sending. But cell broadcast doesn’t support images, which is why cellphone users didn’t get a photo of Rahami in their alerts.

–Who can send out WEAs?

There are hundreds of agencies across the country that are authorized to send WEAs. Of these, the National Weather Service is probably among the most recognizable. That’s the office that sends alerts about tornadoes or flash floods in your area. But depending on where you are, you could receive a WEA from a 911 center, a city government or a sheriff’s department. Only a preapproved agency can send WEAs.

–What steps do they take to send a WEA?

The agency that wants to send a WEA uses an online tool that’s managed and run by the Federal Emergency Management Agency (FEMA). This tool is called IPAWS, short for Integrated Public Alert & Warning System.

Technically, IPAWS is used to distribute alerts over all kinds of media, not just smartphones. This includes the Emergency Alert System (EAS), which is what gets used when you see and hear emergency alerts on TV or the radio. That’s technically a separate system.

–So agencies create an alert with IPAWS, and then IPAWS sends it out as a wireless alert?

Basically. IPAWS passes the alert off to cellular carriers like AT&T and Verizon, who then deliver the WEAs to your phone using their own cell towers.

–Have we used the system like this before?

Well, that’s where things get a bit murky. While many news outlets are reporting that this may be the first time authorities have sent a terror-related alert using the WEA system in particular, FEMA says this is not the first time that IPAWS has been used to notify the public about a dangerous suspect.

–Will we start seeing more agencies use the system this way?

That depends on the situation and the agency involved in generating the messages. New York officials clearly saw fit to use alerts as a manhunt tool, and that may set a precedent. But it’s ultimately up to each jurisdiction to decide.

(c) 2016, The Washington Post · Brian Fung