Austin Police Department interim Chief Brian Manley during a press briefing on Monday made it very clear that Sunday night’s bombing attack that injured two has changed the investigation substantially.

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Manley said that investigators are quite sure the bomb that went off in Austin’s Travis Country neighborhood employed a trip wire. That wire may have been fishing line, a metal wire, filament wire or something else.

In any case, officials said, a wire such as this is tripped via the application of pressure. Someone could detonate it by tripping over it or picking it up.

Manley said the rig demonstrates that investigators are dealing with someone with a high level of skill and sophistication as the trip wire is more advanced than the packages that have exploded in the past three weeks.

That said, Manley said a preliminary investigation shows similarities between the device that detonated Sunday night and the previous three bombs.

About 340 special agents, some of them from San Antonio, are investigating.

Injured Sunday night were a 22-year-old man and a 23-year-old man. Both are white. They apparently tripped the explosive while walking a bike in the neighborhood. Manley said they suffered significant injuries but have been stabilized at a hospital.

While police previously urged residents to be cautious about unknown packages, that warning has now extended to any item that looks out of place. That might include bags, boxes, a suitcase – any item that looks suspicious. People are urged not to approach those items and to call 911. They will be checked out by police and K-9 officers.

“Make sure you are safe and contact us if you see something suspicious,” Manley said.

Manley and federal investigators said there is a great deal of evidence to sift through from Sunday’s blast. Manley urged anyone in the Travis Country neighborhood who may have surveillance footage from Sunday to reach out to APD at 512-974-5210 so that investigators can review it.

Manley said the Travis Country neighborhood will likely remain locked down until 2 p.m. Monday. He said the area is now safe. Investigators held the scene overnight so it could be adequately processed in the daylight. ATF and FBI agents are now conducting post-blast assessment.

Investigators said it is too soon to label the person responsible as a domestic terrorist or classify this as a hate crime. The first three attacks appeared to be targeted. The fourth one appears to be random. Manley did, however, say investigators clearly have a serial bomber on their hands.

“There are innocent people being hurt in this community and it needs to stop,” Manley said.