(CNN) -- In these attacks, it's unclear if the victims were targeted or picked at random.

That's a big reason why a trio of package bombs in the heart of Texas has challenged investigators and terrified a city of almost 1 million people.

Here's what we know about the Austin bombings, and the questions that remain unanswered:

What we know

Two people have been killed: The first blast killed 39-year-old Anthony Stephan House on March 2.

The second blast came more than a week later, on Monday morning, and killed a 17-year-old boy.

The third blast happened around noon Monday and critically injured a 75-year-old woman.

The packages weren't mailed: Instead, they were hand-delivered and left on people's doorsteps.

The boxes looked mundane: The explosives came in "average-sized delivery boxes, not exceptionally large," Austin police Chief Brian Manley said.

What we don't know

Whether the victims were targeted: "The evidence makes us believe these incidents are related," the police chief said. But he said it's not clear the victims knew each other directly, or if they were specifically targeted.

The Washington Post reported that the families of two victims knew each other. House was the stepson of Freddie Dixon, a former pastor at a historic black church in Austin, the Post said. It said Dixon is friends with the grandfather of the second victim, the teenager killed Monday.

Whether that connection played a role remains a mystery.

What types of explosives were used: Police have not detailed what the explosive devices were made from, or what they looked like.

But the chief did say "these are very powerful devices" and "there's a certain level of skill required to move a device like this."

"As you can imagine with a blast like this, the debris field and the evidence really spread over quite a distance, so there's a meticulous process that we go through to collect all that evidence," Manley told CNN affiliate KXAN. "We're using the ATF laboratory to do all the analysis."

What triggered the devices: The devices have exploded at different times after they were discovered.

In the Monday afternoon case, the device exploded immediately after the elderly woman picked up the package.

But in the Monday morning case, a package was brought into the kitchen and opened before it exploded and killed the teen.

What the motive is: The two slain victims were both African-American, and the woman wounded in the third attack is Hispanic. Police have not determined if the attacks are hate crimes but said it's a possibility because of the victims' ethnicities.

What exactly the packages looked like: Authorities haven't said whether the boxes were addressed to anyone specific, or whether there were any other markings.

Who's behind the attacks: So far, no one has claimed responsibility. Texas Gov. Greg Abbott announced a $15,000 reward for information leading to the arrest of the person or persons responsible.