An army of federal agents is on the ground in Austin, Texas, where two men were injured when a bomb exploded Sunday night.

It's the fourth attack in three weeks by a suspect investigators now call a "serial bomber."

The overnight blast left two young men with serious injuries, although doctors say they are not life-threatening.

The string of attacks began on March 2nd when 39-year-old Stephen House was killed when he picked up a package left at his doorstep.

Last week 17-year-old Draylen Mason died and his mother was hurt when they opened a package inside their home, and a 75-year-old victim was severely injured when she moved a package left on her front porch.

500 FBI and ATF agents are helping with the investigation, with more on the way.
     
"The entire national apparatus of the FBI is focused on this, as well as internationally," FBI Special Agent In Charge Christopher Combs said Monday.

The focus is shifting after Sunday night's package bomb was left along the side a road, not a doorstep, and was triggered by a trip wire.

"It's another level of sophistication by the bomb makers," Combs says.  

"That concerns us, the violence is getting worse, instead of better," he added. 

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