By Madison Park, Ralph Ellis and Jason Hanna, CNN

(CNN) -- A series of deadly package bombs in Texas' capital has shaken residents' confidence in one of life's more routine actions -- opening a delivery at home -- as scores of people have called police in the past day to express suspicion about something they've received.

Three package bombs have exploded at homes in Austin over 10 days -- including two Monday -- killing two people and injuring two others.

Investigators say they believe the incidents are related, and residents have responded anxiously. Authorities reported more than 80 calls of apparently suspicious packages Monday evening alone, though subsequent checks revealed nothing alarming, police say.

Daniel Arriaga told CNN affiliate KXAN he called authorities after he found a package he wasn't expecting for his daughter.

"I don't want to give the package to my daughter and something happens, and I would regret it for the rest of my life," he told the station.

Here's what we know so far

• The first blast happened March 2, killing Anthony Stephan House, a 39-year-old African-American man.

• A bombing early Monday killed a 17-year-old African-American male, whom CNN is not naming because authorities haven't identified him yet. A woman was also hurt in that blast with injuries not considered life-threatening, police said.

• Both House and the slain teenager are relatives of prominent members of Austin's African-American community, The Washington Post reported. House was the stepson of Freddie Dixon, a former pastor at a historic black church in Austin, the Post said. Dixon is friends with the grandfather of the teen who was killed Monday, according to the newspaper.

• Another explosion around noon Monday severely wounded a 75-year-old Hispanic woman.

• Police have not decided if these are hate crimes but said that's considered a possibility because the victims are African-American and Hispanic.

• The residents found the packages outside their houses, but none was delivered by the US Postal Service or delivery services such as UPS or FedEx, police said.

• The explosions were not in the immediate vicinity of the ongoing South by Southwest festival, and authorities said the bombings don't appear connected to that event.

The blasts

The packages were placed in front of the residents' houses, the police chief said. They appeared to be "average-sized delivery boxes, not exceptionally large," Austin police Chief Brian Manley said.

The first blast on March 2 that killed House was initially regarded as an isolated incident, but police now classify it as a homicide.

Then Monday, a blast was reported at 6:44 a.m., killing the teenager and injuring a woman.

"One of the residents went out front, and there was a package on the front doorstep," Manley said. "They brought that package inside the residence, and as they opened that package, both victims were in the kitchen, and the package exploded, causing the injuries that resulted in the young man's death and the injuries to the adult female."

Then another blast happened hours later Monday, sending police scrambling from one crime scene to the next.

The latter explosion occurred after the 75-year-old found a package on her porch. When she picked up the package, it detonated. She was taken to the hospital, where she was listed in critical but stable condition, Manley said.

"These are very powerful devices," he said, declining to be specific. "There's a certain level of skill required to move a device like this."

What to do about suspicious packages

Texas Attorney General Ken Paxton warned all Texans to be cautious. "With three reported explosions in the Austin area, I want to urge all Texans to report any suspicious or unexpected packages arriving by mail to local law enforcement authorities. Call 911 immediately if you receive something suspicious," he said on Twitter.

Manley also gave similar advice: "If you've received a package that has been left on your doorstep or left in your yard or left on your driveway that you were not expecting or that was not from someone you know, then give us a call."

Residents seemed to be listening. Some have changed their habits when it comes to handling packages.

Trey Mathis checked to see if there was an address label on one that arrived.

"I took a walking stick and from behind my door, I cautiously tumbled the box over to bring up the label, where I could verify it was addressed to me and from the expected shipper," Mathis said. "I resolved to post the picture onto social media (Instagram, then to Twitter and Facebook) to alert the USPS, if possible."

Local police as well as the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives are working on the case. And Gov. Greg Abbott announced a $15,000 reward for information leading to the arrest of the person or persons involved in the package blasts.

"I want the public to be aware and to be cautious because again we have two cases that are very similar, that have both resulted in a loss of life," Manley said.