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Two men have been injured by an explosion in Austin, Texas, just days after two died in three parcel bomb attacks earlier this month.

Residents of the Texas capital were told by authorities to stay in their homes and avoid touching suspicious packages as FBI agents worked to establish if the four explosions are linked.

Austin police chief Brian Manley said the latest blast on Sunday evening may have been triggered by a tripwire.

He told reporters "it's very possible" that the device "was activated by someone either handling, kicking or coming into contact with a tripwire that activated the device," CNN reports.

"Do not touch any packages or anything that looks like a package. Do not even go near it at this time," Manley told residents.

Austin police meanwhile said on their Twitter feed: "Residents in immediate area ... wait in your homes and follow instructions of officers."

The two victims, thought to be in their 20s, were rushed to hospital after police were called at 8.32pm to be treated for non life-threatening injuries.

The FBI said on Twitter that agents were at the scene of Sunday's explosion in a residential neighbourhood on the west side of the city.

Investigators are still looking for the person behind the three parcel bombs that exploded in three separate east side neighbourhoods of the city, killing two African-American males and leaving a 75-year-old Hispanic woman fighting for her life.

More than 500 federal agents have joined Austin police in the homicide investigation.

Earlier on Sunday, Austin police said whoever was responsible for the bombs was trying to send a message and should contact authorities to explain their motive.

"We are not going to understand that (message) until the suspect or suspects reach out to us to talk to us about what that message was," Manley said.

Manley said police were also investigating the bombings as possible hate crimes.

The first bombing on March 2 killed Anthony Stephan House, a 39-year-old black man. It ripped a hole in a home entrance wall and damaged the front door.

A bomb last Monday morning killed Draylen Mason, a 17-year-old African American teenager and promising musician. It also injured his mother.

A few hours later, a third bombing injured the 75-year-old Hispanic woman, who has not been named.

A reward of $115,000 (£83,000) has been offered for information leading to the arrest and conviction of the person or people responsible.