Austin police appear to have no leads after the fourth explosion by someone they're now calling a "serial bomber," but they hope the public can help. Interim Police Chief Brian Manley on Monday asked residents to hand over their home surveillance footage after Sunday night's blast, reports the Austin American-Statesman. "We are clearly dealing with what we expect to be a serial bomber at this point," Manley said, per CNN. The chief confirmed that Sunday's blast involved a tripwire attached to a package left near a residential road. Two men, ages 22 and 23, remain hospitalized but were expected to recover. The first three bombs were left on doorsteps and appeared to target African-American families, while Sunday night's random victims were white.
Police are working under theory that all four bombs are related, though the latest one showed a higher degree of sophistication with the tripwire. "The device's success, despite significantly different design, further suggests that the bombmaker behind these attacks is an accomplished one, and [is] likely to have received some training, perhaps as a military or police explosive ordnance disposal technician," according to an assessment by global think tank Stratfor Threat Lens. The FBI now has 350 special agents in Austin to help with the investigation, reports NBC News. Local and federal authorities have offered a reward of $100,000, and the state another $15,000. (Read more Austin stories.)