A series of package bombs this week that targeted US Democratic politicians, media and public figures who are unpopular with right-wing activists was probably more of a scare tactic than a murder plot, security analysts said.

Two weeks before the US mid-term elections, experts differed on whether the packages were sent by one person or a group. Police were examining the devices for forensic evidence, which might yield some answers.

None of the 10 bombs exploded and there was no claim of responsibility.

"What they wanted to do was scare people, cause disruption, but not necessarily hurt anybody, because if they'd wanted to hurt people the bombs would have gone off," said Matthew Bradley, a former CIA agent and current regional security director of International SOS and Control Risks.

"My personal feeling is that this is some kind of a wacko, rather than an organisation trying to accomplish something. Evil, mad, has a grudge. A little mix of all of these," said University of Rhode Island chemistry professor, Jimmie Oxley, an explosives expert.

While Oxley said she suspected a single bomber, Bradley said multiple people were probably involved because "it's hard to coordinate that mail delivery on your own."

New York Governor Andrew Cuomo told CNN on Thursday that the more packages authorities find, the faster investigators can track down the perpetrators.

"I would expect more," Cuomo said, adding that the packages may be aimed at intimidating people.

"They are bombs capable of detonation. They did not detonate. Was that purposeful or incidental?"

The thick manila envelopes were intercepted before any reached their intended recipients, who included former President Barack Obama, ex-Vice President Joe Biden and former Secretary of State Hillary Clinton.

eNCA/Reuters