A Texas woman has been arrested in connection with the mailing of ricin-tainted letters to President Barack Obama and New York City's mayor, according to The Associated Press and NBC News.

Shannon Richardson of New Boston, Texas, originally told FBI agents her husband had sent the letters. Officials said investigators found she had sent the letters herself, NBC News reported.

The FBI found "very low concentrations" of ricin on the letters sent to Obama, New York Mayor Michael Bloomberg and a gun control group last week.

In all, at least 10 ricin-laced or suspected ricin-laced letters have been sent to government officials in recent weeks, including Obama.

Ricin is a highly toxic substance derived from castor beans. As little as 500 micrograms -- an amount the size of the head of a pin -- can kill an adult. There is no specific test for exposure and no antidote once exposed.

It can be produced easily and cheaply, and authorities in several countries have investigated links between suspect extremists and ricin. But experts say it is more effective on individuals than as a weapon of mass destruction.

Ricin was used in the 1978 assassination of Bulgarian dissident Georgi Markov. The author, who had defected nine years earlier, was jabbed by the tip of an umbrella while waiting for a bus in London and died four days later.