Preliminary tests on two letters sent to New York City Mayor Michael Bloomberg suggest they contain the deadly poison ricin, say police, according to BBC News.

The writer threatened Mayor Bloomberg, referring to the debate on gun laws, they said.

Emergency personnel who came into contact with one letter showed minor symptoms of ricin exposure.

It is unclear whether there is any link to other threatening, ricin-laced letters sent to public figures.

A Mississippi man is charged with sending letters to President Barack Obama, a judge and Mississippi senator, and another man has been arrested in Washington state over letters sent to a judge.

One thousand times more toxic than cyanide, ricin can be fatal when inhaled, swallowed or injected, although it is possible to recover from exposure.