(CNN) Three package bombs that exploded in Austin, Texas, over 10 days -- killing a teenager and a 39-year-old man and critically injuring an elderly woman -- appear to be connected, police said Monday.

Here's what we know so far:

• Two of the explosions happened within hours of each other Monday, sending police scrambling from one crime scene to the next. The first blast happened March 2.

• The bombs killed a 39-year-old African-American man on March 2, killed a 17-year-old African-American male early Monday morning, and severely injured a 75-year-old Hispanic woman midday Monday. A woman hurt in the Monday morning explosion had non-life threatening injuries, police said.

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

• Police also have not decided if these are hate crimes, but said that's a possibility because of the victims' races.

• "The evidence makes us believe these incidents are related," Austin Police Chief Brian Manley said.

Authorities in Austin investigate a package explosion Monday that left one person dead and another injured.

The packages were placed in front of the residents' houses, the chief said.

In describing the blast that killed the teenager, Manley said: "One of the residents went out front and there was a package on the front doorstep. 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."

Neighbors described house-rattling experiences.

"It shook my house and it shook my body," Isaiah Guerrero told CNN affiliate KXAN of the explosion that injured the elderly woman. Guerrero said he climbed onto his roof, where he could see police walking up to one of his neighbors' homes. "From that angle, you could actually see the house that got the package sent to them."

Anna Marie Castillo said she lives five houses from that blast site. She was at work but quickly contacted her parents, who told her they saw the victim in the yard covered in glass and covering her face with her hands.

Castillo's parents said they carried blankets to her because the explosion ripped off some of her clothing, she said.

"It's really scary and hits really close to home," Castillo said. "I'm worried to look in the mailbox."

Chief urges caution

All residents of Austin should be careful about packages, Manley said. Investigators have not discovered a motive, and he did not say if anybody has claimed responsibility. It's not known if the victims knew each other or if they were targeted, he said.

"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," Manley said.

.@chief_manley advising everyone in Austin to be cautious if they receive a package 📦 and weren't expecting one. If you see something suspicious please dial 9-1-1 so we can send officers out to assist you. #safety #austinpd #atx pic.twitter.com/hA7Tv0bT0E — Austin Police Dept (@Austin_Police) March 12, 2018

Residents seemed to be listening. By 6 p.m. Monday, the police had received 82 calls about suspicious packages, police spokeswoman Stephanie Jacksis told KXAN.

Austin resident Trey Mathis said he expected a package to be delivered Monday but was still nervous when it showed up.

"The odds seemed very favorable, so 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."

Likewise, Manley urged the thousands of visitors in town -- many at the South by Southwest Conference and Festivals -- to be cautious.

"Enjoy yourself, have a good time," he said. "But be aware, be suspicious."

SXSW began Friday in downtown Austin, bringing in thousands of people to the state capital. The explosions were not in the immediate vicinity of the festival and authorities say the bombings don't appear connected to the festival.

"Safety is and always has been a top priority for SXSW," a festival spokesperson said. "We continue to work with law enforcement and our venues to address the safety of SXSW attendees. At this time the Austin Police Department does not believe that the incidents are connected to the event. The substantial security operation already in place for SXSW has been instructed to be extra vigilant."

Teen 'a very happy young man'

Anthony Stephan House

Authorities have identified one victim, Anthony Stephan House, 39, who died from injuries in the March 2 explosion. The woman injured Monday afternoon suffered life-threatening injuries, Manley said.

CNN is not naming the 17-year-old victim because he has not been identified yet by authorities, but an Austin Community College professor who taught him last year said he had a bright future.

"He was smart. He was in high school and taking college classes," US government professor Samuel Osemene said. "He was a young guy with so much future and potential. We talked a lot about college. He hugged me every morning before class."

Osemene said that he would always remember the young man's smile.

"He was a very happy young man with so much energy. He was loving and always had a smile on his face. He made friends easily and will be missed," he said.

The March 2 blast that killed House was reported about 6:55 a.m. in the 1000 block of Haverford Drive. It was first regarded as an isolated incident, but police now classify it as a homicide.

The first Monday blast was reported at 6:44 a.m. in the 4000 block of Oldfort Drive and the second Monday blast was around noon in the 6700 block of Galindo Street.

Police were processing the scene at the first Monday explosion when the second occurred.

'Powerful devices'

Manley said the packages are "average sized delivery boxes, not exceptionally large" that the residents found outside their houses.

"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."

Texas Attorney General Ken Paxton urged 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.

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 9-1-1 immediately if you receive something suspicious. — Ken Paxton (@KenPaxtonTX) March 12, 2018

Local police, as well as agencies including the FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives, are working on the case.

The ATF is processing evidence from the first device at its lab and evidence from the second device will also be sent to an ATF lab for consistency.

Gov. Greg Abbott announced a $15,000 reward for information leading to the identification and arrest of the person or persons involved in the package blasts.

Austin Mayor Steve Adler, speaking Monday at a news conference, said, "This is still a safe city."

"We don't know yet why this is happening or what the motivation is," he said. "Just as soon as we know things and things we can report to the community, I'm going to make sure the community hears it right away."

Correction: A previous headline for this story on the CNN app incorrectly stated that the explosive packages were connected to the South By Southwest Festival, currently taking place in Austin. The police have not said there is a connection. The CNN app headline has been corrected.