Warning follows roadside tripwire bomb that injures two cyclists after deadly parcel bomb attacks killed two earlier in March.
Police and federal investigators in Austin, Texas, have few leads and many concerns after a roadside bomb exploded on Sunday night injuring two white cyclists who came upon a suspicious-looking device.
The bomb was the fourth this month in the area after three previous parcel bomb attacks in mainly African-American neighbourhoods left two people dead and another fighting for her life.
"This device was a device that was activated by someone either handling, kicking, or coming in contact with a tripwire and that is something that's very important for this community to understand so that we can all remain safe while we work through this investigation," said Austin's police chief Brian Manley.
"The reward in this case has gone up to 115,000 dollars now being offered for information that leads to an arrest here. The investigation continues, that's the latest from Austin," reports NBC's Jay Gray.
Austin, the state capital, is a university town and considerably more liberal than most of the rest of Texas.
On Saturday a concert in Austin, part of the renowned SXSW festival, featuring African-American artists headlined by The Roots had to be cancelled after a hoax bomb threat. Police have since arrested a suspect for making the call.