A police crime scene van arrives near the site of Sunday's explosion in Austin, Texas. Picture: AP Photo/Eric Gay

Washington - A package believed to be bound for Austin exploded at a Texas FedEx facility overnight, law enforcement officials said early Tuesday.

The explosion occurred at a facility in Schertz, Texas, just northeast of San Antonio sometime around 1 a.m., said FBI Special Agent Michelle Lee.

ATF spokesperson Nicole Strong said that early indications are that no one was injured.

A law enforcement source said that police are investigating whether the explosion was related to the other four in Austin, and that it "is definitely a concern of ours."

The incident happened as investigators scrambled for clues at the scene of another mysterious explosion Sunday in Austin, the fourth in a string of attacks in the city this month. Authorities believe they are the work of a sophisticated "serial bomber" who has been terrorising Austin with increasingly complex devices.

Read more: Police: Two deadly Texas package bombs are linked

Parcel bombs have been used before in apparent terrorist-linked plots attempting to use delivery services such as FedEx. In 2010, two packages containing plastic explosives and a detonating mechanism were found during transit on two cargo flights bound for the United States. One was discovered in Dubai, and the other at East Midlands Airport in Britain. The explosive devices - hidden in printer cartridges - were originally shipped from Yemen. Later, al-Qaida in the Arabian Peninsula claimed responsibility.

Investigators in Texas have not linked the blasts there to any particular individual or organisation.

Sunday's blast in Austin, which injured two men walking through a residential area, marked an escalation in both the tactics and skills displayed by the bomber or bombers, police said. While the three previous devices were hidden in packages delivered to homes in residential neighbourhoods, the fourth device - anchored to a for-sale sign - was left on the side of the road and was rigged with a tripwire, showing "a higher level of sophistication, a higher level of skill," said Brian Manley, the interim Austin police chief.

"What we have seen now is a significant change from what appeared to be three very-targeted attacks to what was, last night, an attack that would have hit a random victim that happened to walk by," Manley said at a news briefing Monday. "So we've definitely seen a change in the method."

Also read: Austin package bombings: US police scramble for motive

Investigators were just beginning to probe the reported explosion at the FedEx facility. So no information was available about the type of blast or device involved.

The fourth device in Austin had "similarities" to the three bombs that detonated in the Texas capital in the past three weeks, killing two people and injuring two others, one seriously, Manley said. 

The explosion Sunday night plunged the city further into a frightening uncertainty that has left residents on edge. Authorities have seemed vexed, completely at a loss to explain who could be setting off the devices - or why - and asking the unknown attacker to communicate with them. They also have offered rewards of up to $115 000 for information in the case.

The first three explosives all hit eastern Austin, affecting areas where black and Hispanic residents live, prompting some to question whether the initial blast on March 2 would have prompted more urgency had it gone off in a more affluent, predominantly white neighbourhood - like where the attack Sunday night happened, injuring two white men.

"This is a public threat," Nelson Linder, president of NAACP's Austin chapter, said Monday. "Now that the geography has changed, it's going to widen people's perspectives. Nobody can take this lightly; we're all vulnerable."

Linder added: "Like they tell us in the military, when you walk, look down at where you're walking."

Read more: Austin bombings: 'Explosion may have been triggered by trip-wire'

Police have declined to describe the bombs in detail, saying only that they appear to be the sophisticated work of a person or people who know what they are doing - and noting that the bombmaker has been able to assemble and deliver the devices without setting them off. After telling residents to remain wary of unexpected or suspicious packages, authorities now are urging broader caution.

"We're even more concerned now that if people see something suspicious, they just stay away from it altogether and contact law enforcement," said Fred Milanowski, special agent in charge of the Houston division of the Bureau of Alcohol, Tobacco, Firearms and Explosives. "Because if they move that package or if they step on that tripwire, it's likely to detonate."

Milanowski said bombs using tripwires are activated when any pressure is applied to the wires, and he said that can include people "tripping over it or picking up the package."

Investigators have said the attacks might have been motivated by racial bias, since the previous explosions killed two black people and wounded a Hispanic woman, though they acknowledged that this is just one theory. The explosion Sunday injured two white men - one 22, the other 23 - walking through Travis Country, a wealthy neighborhood in southwest Austin, far from the other three attacks.

The men were taken to a hospital with serious but non-life-threatening injuries, and Manley said they were in stable condition Monday.

"With this tripwire, this changes things," said Christopher Combs, special agent in charge of the FBI's San Antonio office. "It's more sophisticated. It's not targeted to individuals. We're very concerned that with tripwires, a child could be walking down the sidewalk and hit something."

Combs said the bureau had brought in more than 350 special agents to help with the investigation, describing it as an "unprecedented response."

Dan Defenbaugh, a former FBI official who spent decades investigating bomb cases, said the use of a tripwire Sunday "dramatically changes the tactics involved."

The shift in method - from a bomb targeting an individual or their family to a device that could kill a random passerby - evokes past attacks that created climates of fear, such as the Washington-area sniper shootings that gripped the D.C. region for weeks in 2002 as victims were shot at random. And, as during the Unabomber's campaign of sending explosives through the mail, the Austin attacks thus far defy explanation and have come without any threat or warning.

"Once a bomb builder makes a device, they usually make it the same way each and every time," said Defenbaugh, who managed the Oklahoma City bombing investigation. "That's not happening here. I'm also troubled by the fact that there hasn't been an extortion demand or communication."

Defenbaugh praised federal law enforcement agencies for flooding the Austin area with hundreds of agents, but said all those personnel will need to be closely managed to ensure key tips don't slip through the cracks. In the Oklahoma City bombing case, he noted, investigators gathered 1.8 million hotel records alone. The FBI is sending profilers from Quantico, Va., as well as investigative specialists and emergency response personnel from the surrounding areas.

"It's a huge, complex effort," Defenbaugh said, adding that investigators have two things working in their favor so far. First, ATF bomb experts are examining the remnants of the devices, which could offer significant clues to how they were made and by whom. Second, he said, no details of the bombs' construction have become public, which enables investigators to quickly eliminate any tips involving bombs made using different materials.

"I'm sure it's on the minds of a lot of law enforcement that they have a serial killer on their hands," he said.

Many in the neighborhood where the fourth device detonated said they felt removed from the terror that had shaken other parts of town - until Sunday.

"It appears that no one is safe, and I'm very fearful for our community," said Richard Herrington, 75, who was watching the NCAA men's basketball tournament when he heard the explosion Sunday night. "It's very concerning that this person is becoming more sophisticated."

Ellen Troxclair, an Austin City Council member who represents the district where the Sunday explosion occurred, said she was shocked by seeing the blast there.

"The tripwire definitely instilled some fear into this neighborhood," she said. "They just want to know what's going on."

When the blast detonated Sunday, Eliza May said she heard a sound akin to "when the generators go out, but like five times louder." May said she was ordered to stay in her home Monday morning and was given permission to work there, but she found herself unable to concentrate.

"This thing is overwhelming," she said. "My house is a crime scene. I can see the FBI right now - they're in hazmat suits, walking in a line down the street."

The explosions have effectively occurred in a ring outside of Austin's core, which includes downtown, the Texas capitol and the University of Texas. The first two bombs were both deadly: Anthony Stephan House, a 39-year-old construction worker, was killed March 2. Then, 10 days later, Draylen Mason, a 17-year-old high school student, was killed in an explosion that injured his mother.

House and Mason were both related to prominent members of Austin's African American community, and they had family members who are close, raising fears that their race or familial connections might have played a role in the attacks.

Just hours after Mason was killed, a third bomb went off, seriously injuring Esperanza Herrera, a 75-year-old Hispanic woman who was visiting her mother. But that blast puzzled investigators because it was addressed to a different home and apparently exploded when Herrera was carrying it, according to two people familiar with the case.

Police had initially described the explosion that killed House as an isolated incident, though they changed course 10 days later when the additional bombs detonated. Manley said Monday that police still do not have evidence leading them to a particular suspect, and he reiterated his plea to the public for tips and information.

Austin Mayor Steve Adler said in a telephone interview Monday that "with each additional event, the horrible part is that people are getting hurt." But, he added, "it also means that law enforcement folks get additional forensic evidence."

The explosion Sunday went off just hours after the Austin police made a public appeal in the case, increasing the reward for information to $100 000 and addressing the bomber or bombers in particular. Republican Gov. Greg Abbott has also offered another $15 000 for information.

"These events in Austin have garnered worldwide attention," Manley said during the earlier announcement. "And we assure you that we are listening. We want to understand what brought you to this point, and we want to listen to you."