>> Analyze. We are clearly dealing with what we expect to be a serial bomber at this point.>> Chilling words from Austin's police chief on Monday after a fourth bomb exploded in the city this month, injuring two men out for a walk. The belief that Sunday night's blast was set off by a trip wire, a more advanced design than previous explosions has authorities issuing stronger warnings to an already frightened public.
>> Like the chief said, we've even more concerned now that people see something suspicious, they just stay away from it all together and contact law enforcement. Because if they move that package or if they step on that trip wire, it's likely to detonate.>> Reuters Correspondent John Herskovich is on the story.
>> This is caused fear among residents that they could be next. There are worries about suspicious packages. Parents have been warned. The city officials have been warning people, but this idea about a trip wire is causing even more fear. Because it could people walking their dogs out and about.
>> Over 500 federal agents, including the ATF and bomb technicians have joined the investigation. Hoping to find the person or people behind three recent parcel bombs that exploded in three separate neighborhoods. The blast killed 2 black men and left a 75-year old Hispanic woman fighting for her life.
The bombings are being investigated as possible hate crimes or terrorism. A $115,000 reward has been offered for information leading to the arrest and conviction.