A 41-year-old martial arts instructor on Friday pleaded guilty to sending ricin-laced letters to President Barack Obama and other officials, the United States Justice Department said.

James Everett Dutschke triggered widespread alarm last April after the emergence of the potentially lethal letters, as the United States reeled in shock from the Boston Marathon bomb attack.

The incident carried an eerie echo of the anthrax letters sent to public officials shortly after the September 11, 2001 attacks.

Appearing in a federal court in Oxford, Mississippi on Friday, Dutschke pleaded guilty to charges of developing and possessing ricin, and mailing ricin-impregnated letters, including one that threatened Obama.

According to the terms of his guilty plea, Dutschke has agreed to a 25-year jail term. His sentence will be formally confirmed by federal judge Sharion Aycock in two months time.

The Justice Department said Dutschke had used the Internet to research how to produce and use ricin, a biological agent and toxin which can be lethal in only the tiniest amounts.

Dutschke purchased castor beans or seeds -- from which ricin is made -- via the Internet before buying tools to process and develop the poison, such as latex gloves, grinders and masks from vendors in the area.

He then sent three ricin-laced letters to Obama, a US Senator, and a Mississippi Justice Court Judge.

Authorities had initially arrested another man in the aftermath of the letters' emergence: Elvis impersonator Paul Kevin Curtis.

However Curtis's attorney insisted his client had been framed, and suggested Dutschke was responsible.

Dutschke was also known to have had a long-standing dispute with the Mississippi justice who received one of the letters.

Dutschke was arrested on April 27 last year before being indicted in June.