Three explosions ripped through Brussels' airport and subway Tuesday, killing 31 people and injuring 330 others.
Belgian authorities quickly identified four people they said were involved. One survived and fled, leading police to search for him and a possible associate in a massive manhunt across Europe. They have any number of tech tools they could use to catch his digital trail.
The science of searching out a dangerous suspect has changed dramatically over the past two decades, and technology now records clues and crunches information faster than the best paper-pushing police officer ever could.
But while digital evidence can speed up a search, agencies still rely on tips from human beings. To help with that, there's sophisticated technology that can scan social media for clues, examine images for leads near suspects' neighborhoods, and combine communications from different law enforcement agencies into a single feed of information.
Undoubtedly, the police are poring over video images captured by security cameras inside Brussels' airport and subway stations and on city streets. Such footage has already helped investigators find an undetonated bomb. A taxi driver who saw a surveillance photo pointed authorities to a house the terrorists used to make bombs.
And on Tuesday, the UK's Metropolitan Police published a website where people who were on the scene can upload footage. Later that day Belgian police said they found an email address from similar submissions.
The police haven't said precisely which software or methods they're using beyond the obvious. The Belgian Ministry of Justice can't comment on the ongoing investigation, spokesman Edward Landtsheere said. And the federal prosecutor's office didn't respond to requests for comment.
But we do have some clues.
Faces in a crowd
Facial recognition is one of the most advanced tools law enforcement has at its disposal. It's good enough to digitally remove the suspect's hat and sunglasses that we see in the surveillance footage, said Todd Butler, a vice president at Chenega International Consulting, which sells software to government agencies.
That digital image can then be fed through any image database, regardless of source. The software is a lot better at matching faces than the human eye, which is right about half the time.
"The probability of a match goes up significantly with facial recognition, to about 80 or 90 percent," said Butler.
Crowdsourcing the manhunt
Law enforcement agencies keep a close eye on social media feeds when dealing with terrorists, said FBI spokesman Prentice Danner III, who agreed to talk about law enforcement's tech tools in general, but not on the current manhunt in Europe.
"We get a lot of tweets," said Danner, who helps run the FBI's Twitter account for the San Francisco field. "Some are valuable; some are not. Using technology to weed through those is useful."
For proof, just consider what happened on Reddit in 2013, when redditors fingered three innocent people as the Boston Marathon bombers.
"It can take you the wrong way. It can be malicious," Eugene O'Donnell, a lecturer at the John Jay College of Criminal Justice, said about social media chatter. Even so, tips from the Internet are worthwhile. "That's the price you have to pay."
Too much information
Technology has a downside when it comes to tracking down suspects, said Butler, who previously worked as an intelligence analyst. "There's just too much data."
That's why law enforcement agencies need a single information feed that puts everything in one place, from facial recognition clues to social media data and local alerts. The FBI used that kind of software to secure the Super Bowl in Santa Clara, California, earlier this year.
But technology does have its limitations.
"The mind is what we can't get a read on," said O'Donnell. "We can track people's movements. We can hear what they say, but we can't get into their hearts and minds." Still, he said, "We can get into their minds more than we used to."
And for the immediate future, said Butler, no technology can replace the most important tool we still have: the brain of a well-trained professional.
"Nothing's going to replace that trained person who's going to get the last 10 percent of the data."
CNET's Richard Nieva contributed to this story.