Police are investigating a manifesto which indicates the California synagogue shooter may have been inspired by the Christchurch mosque attacks.

One woman is dead and three other people are injured after a man opened fire in the synagogue in Poway near San Diego.

Police have 19-year-old John Earnest in custody and say he's also being investigated over a fire at a mosque in a nearby city last month.

An open letter posted online in his name references other attacks on houses of worship, including the attack on the Tree of Life Synagogue in Pittsburgh and the mosque shootings in Christchurch.

Police are investigating its validity.

However, one terror analyst says the depths of the internet are making angry young men all the more dangerous.

Intelligence analyst Paul Buchanan told the Weekend Collective we are talking about people who want to find justification for their unhappy, rotten lives.

"And in the old days., they would have to go up and meet up with like minded spirits physically. Today, you can do all that online in the confines of your bedroom." 

The news comes as it is revealed that New Zealand police have put together a watchlist put following the Christchurch terror attack, which left 50 people dead and as many injured. 

More than 100 people are reportedly being actively monitored by police.

People on the list include white supremacists, people "disaffected" with firearm licences, and others with racist and radical views are also included.

Buchanan told the Weekend Collective it's a very good step.

"Police have decided to modify their profiling of potential terrorist suspects, and rather than looking at communities, such as the Muslim community, because there are a few bad eggs in it, they are now looking at behaviours, including online behaviour."

Buchanan says that of this list of people, there could be people that are simply ranters.

"This leak could serve as a warning to them that they better back off, as they have now attracted attention." 

He believes that in general we are quite safe, but the gunman accused of the attack is part of a larger community. 

"We seem to be seeing a backlash from alt-right, neo-Nazi, white supremacists around the world, and New Zealand is not immune from that."

Despite this, Buchanan says that there is a slim chance of a revenge attack.

Police aren't commenting on the list, but urge anyone who has concerns about someone to report them to their local police station or Crimestoppers.