Austin, Texas (CNN) Austin police just confirmed what residents have feared for weeks -- a suspected serial bomber is attacking their city.

For the fourth time this month, a device exploded on Austin residents. What makes Sunday night's blast especially terrifying is that the device was left on the side of a residential road and triggered by a tripwire, police said Monday.

Investigators have found similarities between that device and three previous bombs, which were stuffed inside packages and left on residents' doorsteps, Austin Police Chief Brian Manley said.

"We are clearly dealing with what we expect to be a serial bomber at this point," he said.

The latest device -- which was triggered by a tripwire -- shows "a higher level of sophistication, a higher level of skill," he said.

And unlike the victims of the previous blasts, the two men wounded in Sunday's explosion are white, Austin police said. Both men are expected to recover.

Latest developments

Police departments in Houston and San Antonio are sending bomb technicians and canine teams to Austin to help in the investigation, their police chiefs said Monday.

President Donald Trump has been briefed on the Austin bombings, a White House spokesman said Monday, adding the White House pledges its support to local law enforcement.

Three members of the Congressional Black Caucus called Monday for federal officials to classify the bombings as terrorist attacks and determine whether they are "ideologically or racially motivated."

Texas Gov. Greg Abbott announced more than $265,000 in emergency funding for police to purchase seven portable X-ray systems for use in detecting bombs and investigating suspicious packages.

The Travis Country neighborhood of Austin was on lockdown Monday, Manley said, as authorities investigate the pieces of evidence strewn across a wide area.

Manley asked anyone in the neighborhood with security camera footage to call police.

The Austin public school district says it couldn't send buses to the affected neighborhood Monday because of police activity.

An indiscriminate wake-up call

Many minority residents in Austin have been on edge since the bombings started, as the first three bombings killed or wounded minorities.

Several residents under lockdown Monday said they were stunned the latest attack happened in their neighborhood.

Unlike the first three bombings, which happened in east Austin, the latest attack happened in a predominantly white part of town.

Eliza May said because the first three bombings happening on the east side of the city, in predominantly minority areas, she hadn't been following the news closely.

May said that as a resident of the affluent Travis Country neighborhood , she assumed she had nothing to worry about.

"We feel safe. This isn't something that you'd expect around here," May said.

Now, "it's obvious that you have to be alert."

"This was a random bomb," she said. "This could have been any one of us."

Neighbor Shonda Mace said the bombing "is going to be (a) life-changing event for our neighborhood."

"I'm scared about what's going to happen next," she said.

Tripwire could give clues

"The use of a tripwire is far less discriminating than leaving parcel bombs at residences and suggests that the latest victims were not specifically targeted," the global think tank Stratfor Threat Lens said.

"The device's success, despite significantly different design, further suggests that the bomb maker behind these attacks is an accomplished one, and has likely to have received some training, perhaps as a military or police explosive ordnance disposal technician."

"As the bomb maker changes up design and geography, all residents of Austin and surrounding areas should avoid suspicious items," Stratfor Threat Lens said.

'Extra level of vigilance' needed

The latest blast came less than a week after police said the three previous explosions -- in a span of 10 days -- were connected. Those blasts killed a man and a teenager, and wounded two others.

Police are working under the belief that the explosions are related. Manley said they'll get a better idea with a post-blast analysis and examination of the device components.

In the meantime, Manley told residents not to touch or go near anything that looks suspicious.

"We now need the community to have an extra level of vigilance and pay attention to any suspicious device -- whether it be a package or a bag, a backpack -- anything that looks out of place," Manley said Monday. "Do not approach items like that."

APD is asking the public to remain vigilant and report anything suspicious. If you come across ANYTHING that looks suspicious, DO NOT touch, handle or disturb it. Keep a safe distance and call 9-1-1 immediately. pic.twitter.com/buJsLqeRy5 — Austin Police Dept (@Austin_Police) March 19, 2018

More resources for police

Texas Gov. Greg Abbott announced the release of $265,500 in emergency funding Monday for the Austin Police Department and the Texas Ranger Response Team to purchase seven portable X-ray systems.

The systems can be used at the scene and provide "clear visual evidence for rapid assessment of a package's safety," the governor's office said.

"I want to ensure everyone in the Austin region and the entire state that Texas is committed to providing every resource necessary to make sure these crimes are solved as quickly as possible," Abbott said in a statement.

Other Texas law enforcement agencies are contributing resources as well.

The Houston police department said Monday it's sending several teams to Austin to help with the investigation. They include bomb technicians, an explosive ordnance disposal team, and an explosives-trained dog and handler, Chief Art Acevedo said Monday.

"We stand with the Austin Police Department," Acevedo said.

The San Antonio Police Department dispatched two bomb technicians and a bomb dog to Austin on Monday morning, Police Chief William McManus said.

Shortly before the fourth bombing, the reward for information leading to the arrest of anyone responsible for the three blasts increased to a total of $115,000, authorities said.

Officials have urged residents to call police with any tips they may have, even if the information seems to be "inconsequential."