A police department in Austin, Texas, has received over 150 calls from concerned residents about unexpected packages they have received in the mail over fears that they may be bombs, CNN reports. 

The calls are coming in after three separate packaged bombs exploded in the city in recent days, killing a 17-year-old boy and a 39-year-old man. Two other people were injured in the explosions. 

Austin Mayor Ken Paxton told everyone to call 911 immediately if they see any suspicious behavior. 

Police Chief Brian Manley echoed the same 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," he said.

Residents are taking it to heart. Trey Mathis posted a picture on Instagram saying he had to flip the package over with a stick to check the label before opening it. 

"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," Matthis told CNN. 

One man who called in a package said that he did so out of concern for the safety of 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 CNN. 

After investigating the remnants of the bombs, police have concluded that the devices are not ordinary. Manley said Tuesday that it took "a level of skill" to construct the three bombs that have already exploded. 

"There's a certain level of skill and sophistication that whoever is doing this has, and … we are hoping to use the evidence we have to track them down based on what we are seeing on all three scenes that seem to be consistent," he explained. 

Although all of the victims of the attacks have been minorities, police are currently holding off on declaring the attacks hate crimes; the search for the bomber is ongoing.