AUSTIN, Texas (AP) -  Austin's mayor says growing anxieties are "legitimate and real" following the fourth bombing in the city this month.

Mayor Steve Adler told The Associated Press on Monday that residents shouldn't think twice about calling 911 if they see anything suspicious. A blast occurred Sunday night in a neighborhood far from the sites of the three package bombings in Austin this month.

Police haven't confirmed the latest explosion, which injured two men who were riding or pushing bikes, is related to the first three, but they're looking into the possibility. Authorities say Sunday's explosion was detonated by a tripwire and showed a different level of skill from previous blasts. Adler says the concern now is that "the methodology has changed."

The #FBI, ATF, and Austin Police Department seek the public's assistance with identifying the individual(s) responsible for the package bombs which injured and killed several Austin, Texas, residents. Reward offered: https://t.co/fcGIut3Ou8 pic.twitter.com/wjdbWZciQZ

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