A woman has been indicted on charges that she sent ricin-laced letters to President Barack Obama and New York Mayor Michael Bloomberg.

Federal prosecutors said the alleged plot was to frame her estranged husband.

Shannon Richardson is charged with two counts of mailing a threatening communication and one of making a threat against the US president. 

The Texas actress is also charged with sending the threatening letters to a third man who heads the mayor's gun-control group.

She faces up to five years in prison on each of the charges if found guilty.

Prosecutors accuse Ms Richardson (35) of mailing the letters and trying to pin the crime on Nathan Richardson who she married in 2011.

He filed for divorce earlier this month.

He told a newspaper that he contemplated divorce last year but reconsidered when the relationship seemed to improve.

The marriage was at least the third for Ms Richardson, who has five children ranging in age from four to 19 from other relationships, according to Nathan Richardson's attorney. 

A federal judge last week ordered the accused to undergo psychological tests.

Her lawyer had requested that she be examined, saying she had displayed "a pattern of behaviour" that calls into question whether she could assist in her defence.

Authorities have determined that the ricin letters - which threatened violence against gun-control advocates - were mailed from New Boston, about 240 km north-east of Dallas, or nearby Texarkana and postmarked in Shreveport, Louisiana.

According to an FBI affidavit, Ms Richardson first contacted authorities to implicate her husband in the scheme.

But she failed a polygraph test and investigators found inconsistencies in her story, the document alleges.

Ms Richardson later admitted she mailed the letters but maintained that her husband made her do it, according to the affidavit.

The actress has had minor roles on television and in films under the name Shannon Guess.