AUSTIN, Texas — Fear escalated across Texas’ capital city on Monday after the fourth bombing this month — a blast that was triggered this time by a tripwire, demonstrating what police called a “higher level of sophistication” than the package bombs used in the previous attacks.

Two men were wounded by the explosion Sunday night as they walked along a street and remained hospitalized in stable condition. The three earlier bombings since March 2 involved parcels left on doorsteps and killed two people while wounding two others.