Image copyright Reuters Image caption Police have sealed the area around the scene of the blast

Police in Austin, Texas, have said four explosions that have rocked the city, killing two people, are believed to be the work of a serial bomber.

The latest explosion on Sunday night, which injured two white men, aged 22 and 23, may have been activated by a tripwire, police say.

Three parcel bombs left on doorsteps have killed two people and injured two others since early March.

Investigators say they have seen similarities in all four cases.

"We are clearly dealing with what we expect to be a serial bomber at this point," Austin police chief Brian Manley said at a news conference on Monday.

"We have seen similarities in the devices that exploded here last night and the other three devices."

Sunday night's victims suffered non-life threatening injuries and were taken to hospital after encountering a suspicious device on the side of a road in a residential area, police said.

The latest blast came hours after officials announced a new $100,000 (£71,000) reward for information.

The cash reward is on top of an existing $15,000 being offered by the state's governor.

The two men killed earlier this month were African Americans and police have not ruled out racism as a possible motive.

Mr Manley has said he believed the attacks were "meant to send a message".

He said authorities could not confirm whether a specific ideology was behind the attacks, but appealed to those behind the bombings to contact authorities directly.

"We are not going to understand that [message] until the suspect or suspects reach out to us to talk to us," Chief Manley said.

Image copyright FBI

Hundreds of federal agents are helping local police, but there has been no arrest. Authorities say they have responded to 735 reports of suspicious items since last Monday.

The first bomb exploded on 2 March, killing Anthony Stephan House, 29, at his home. Two more bombs exploded 10 days later, and were linked to the initial blast.

Draylen William Mason, 17, was killed when he brought a package inside his home. The explosion also critically injured his mother.

Hours later a 75-year-old Hispanic woman was injured by another package. Local media reports this may have been intended for someone else.

The local National Association for the Advancement of Colored People (NAACP) president Nelson Linder told NBC news that the two murdered victims had links to local prominent African-American families and were connected by the same Methodist church.

Investigators say the devices were being left on doorsteps overnight, not being delivered by official methods.

With the city on high alert, a concert by hip-hop band the Roots was cancelled on Saturday after organisers received a bomb threat via e-mail.

They were due to perform at the city's South by Southwest festival, known as SXSW.

A 26-year-old was later arrested, but police have ruled out any link to the package bombs.