A fourth bomb this month went off in Austin, Texas Sunday night, March 18. Officials are speculating that this bomb is affiliated with the three other bombs that went off in the Texas capital a few weeks prior.

Authorities surmise that it is a “serial bomber” that is responsible for the bombings.

Two men were injured last night while walking on the street in a neighborhood. Citizens and residents were instructed to stay locked in their homes until officials had more information.

A tripwire on the road was set off by the men, leading officials to believe that the serial bomber is “a higher level of sophistication and skill,” said Brian Manley, the interim Austin police chief to the Washington Post.

According to officials, the three previous bombs were disguised in packages left on doorsteps, but this new tripwire gives more dimension and speculation to the situation.

There are no leads or information given about the serial bomber at this time.