By Tina Burnside, Ralph Ellis and Joe Sterling, CNN

(CNN) -- Three explosions that occurred when people picked up packages outside their residences appear to be connected, Austin, Texas, city Police Chief Brian Manley said Monday at a news conference.

A Monday morning explosion killed a 17-year-old African-American boy and also injured a woman at the house. A second Monday explosion left a 75-year-old woman in critical condition, the chief said. A March 2 blast killed a 39-year-old African-American man.

All the explosions occurred when people who lived at the residences went outside and picked up packages, Manley said. The packages were not delivered by the postal service or delivery services like UPS or FedEx, he said.

"The evidence makes us believe these incidents are related," he said, saying investigators have not come up with 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.

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

"We're not ruling anything out at this point," he said. "We're willing to investigate any avenue."

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

In describing the Monday morning blast that killed the teenager, the chief said: "What we understand at this point is that early this morning is that 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."

That woman's injuries were not life-threatening, he said.

Manley strongly urged residents to be on the lookout for suspicious packages and to alert authorities immediately.

"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.

Likewise, he urged the thousands of visitors in town -- many at the South by Southwest Interactive Festival -- to be cautious. "Enjoy yourself, have a good time," he said. "But be aware, be suspicious."

The festival began Friday in downtown Austin and ends Tuesday. The explosions were not in the immediate vicinity of the festival.

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.

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.