The package that appeared on the front porch of a leafy northeast Austin home on March 2 gained little special notice, in part because receiving a package is such a regular and frequent occurrence across the United States that most people wouldn’t think twice about retrieving it. In a culture that increasingly shops online, such packages are routine.

This one was not. Its contents exploded with a force strong enough to kill 39-year-old Anthony Stephan House and alarm many of his neighbors. At the time, authorities said the blast was “suspicious” but likely “an isolated incident” that posed no ongoing danger to the community.

On Monday, 10 days later, a pair of packages left at two homes in other parts of Austin also detonated, killing a 17-year-old boy and seriously injuring two women. Police quickly linked the attacks to an unknown, sophisticated bomb-maker who had turned to a rare implement of death, transforming the mundane act of picking up a package off the porch into a terrifying ordeal.

Austin Police Chief Brian Manley warned residents numerous times to report suspicious or unexpected packages.

“DO NOT open it, call 911 immediately,” he tweeted.

The bombings called to mind the crimes committed by Theodore “Ted” Kaczynski — also known as the Unabomber — who during a 17-year period targeted more than a dozen victims with explosives sent through the mail. In all, his mail bombs killed three people and injured 23.

After a lengthy manhunt, the FBI finally captured Kaczynski in 1996. In the 22 years since, mail and package bombs have largely slipped from the public eye.

Until the Austin explosions.

The warnings Monday seemed especially foreboding in a time when package deliveries, once expected only on special occasions, are now ubiquitous. The rise of Internet shopping — with promises to be able to ship an increasing array of items to your doorstep, from toilet paper to potato chips to computers, sometimes within hours — has threatened to overwhelm postal workers, college campuses and rural towns. (Jeffrey P. Bezos, the chief executive of, owns The Washington Post.)

Delivery methods have evolved to include not just the U.S. Postal Service but also UPS, FedEx and even independent contractors driving their own vehicles, from white vans to nondescript sedans. Someone dropping off a package in street clothes, driving an unmarked car, is unlikely to draw much attention.

“Decades ago, the majority of packages were shipped through the U.S. Postal Service,” said Michael Bouchard, former assistant director of field operations for the Bureau of Alcohol, Tobacco, Firearms and Explosives. “Now you have all kind of delivery services,” and people by nature typically open packages that arrive at their homes.

“There’s certainly more packages at my door than I’ve ever seen before,” Bouchard said. “It’s just second nature now: ‘Okay, here’s a package, my name’s on it.’ Even if you don’t remember ordering something, if it comes in an Amazon box or whatever box, it looks perfectly fine.”

Previous telltale signs of a suspicious package — stereotypical oil stains, protruding wires or extra postage on a box — no longer necessarily apply to modern explosive devices, Bouchard said.

“There’s so many different types of explosives and chemical mixtures you can use, and you don’t put stamps on things anymore, they’re postage labels,” Bouchard said, noting that it would be difficult for an average homeowner to tell that what they have is dangerous because packages are “so common and you can just disguise anything in a box you commonly get at your door.”

Suddenly, every parcel outside an Austin home became a potential threat this week. By Tuesday morning, authorities in the city had received approximately 150 calls to report suspicious packages, though none turned out to be dangerous, they said.

Police in Austin have divulged little about the three bombs, except to say they arrived at the homes as “box-type deliveries” and included powerful devices that had been constructed with “a certain level of skill and sophistication.” In the first and third bombings, the explosions took place outside the homes. In the second incident, the package was moved inside before it detonated and killed a teenager, police said.

Police said they didn’t yet know if the victims were the intended targets, or how the homes were selected for attacks.

The challenge for law enforcement, Bouchard said, will be trying to determine a motive when “anybody” could make a bomb given what’s available online. (Last fall, Amazon said it would review its algorithms after the website automatically suggested bomb-making components to customers as items “frequently bought together.”)

“There’s certainly enough information on the Internet,” Bouchard said, citing “thousands” of websites and YouTube videos detailing how to put together an explosive — though he warned against searching for them.

“I wouldn’t encourage anybody to go there because the government may be monitoring who’s looking at bomb-making websites,” he said. “If you’re just curiously looking, you may get a knock on your door someday.”

Exploding package bombs are unusual, and reported suspicious or unattended package incidents are declining despite the vast increase in package shipments, according to the Bureau of Alcohol, Tobacco, Firearms and Explosives. In 2016, the federal law enforcement organization recorded 939 incidents related to packages and parcels, a significant decrease from 1,739 the year before.

By comparison, incidents involving backpacks and purses increased 57 percent in 2016. Figures from last year are not yet available, an ATF spokesman said.

Bouchard said he isn’t sure why there has been a decrease but said that, from a criminal’s perspective, trying to inflict harm using package bombs can be iffy. They can detonate prematurely, or not at all, and they might never make it to their intended target because of screening measures the Postal Service and shipping companies use.

“You risk a lot with the way they’re handled,” Bouchard said, noting that packages can be delayed or sent to the wrong place.

Representatives for the U.S. Postal Service and UPS said they could not divulge what security measures they have in place to screen and flag suspicious packages. A FedEx spokeswoman declined to answer questions about package security.

UPS spokesman Matthew O’Connor encouraged customers to track their packages online so they know when to expect them at their doorsteps and to always look out for UPS-branded uniforms and delivery trucks.

“Our drivers wear the UPS uniform and announce themselves when making deliveries by knocking on the door or ringing the doorbell,” O’Connor said.

Those who send explosives through the mail usually are trying to achieve one of two things, Bouchard said.

“From what I’ve seen, the majority of package bombs were specifically targeted toward an individual, whether it’s the breaking up of a relationship or someone who burned someone in a business deal,” Bouchard said. “Or they’ve got a broader target. They’re sending a message.”

Mark Berman contributed to this report.

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