A US woman who was held in Ireland in connection with an alleged murder plot announced she had converted to Islam and told her family they'd go to hell if they didn't follow in her steps, her mother said.

Jamie Paulin-Ramirez, 31, also began talking about Jihad with her Muslim stepfather and spent most of her time online as she withdrew from her family, Ms Mott said.

"We were enemies," Christine Mott said. "We couldn't even speak to each other."

Last year, on September 11, Ms Paulin-Ramirez left Leadville, Colorado, an old silver mining town west of Denver that was Colorado's second-largest city during its heyday. She took her six-year-old son with her, her mother said.

A US official, who was not authorised to discuss the investigation and spoke on condition of anonymity, said Ms Paulin-Ramirez was detained in Ireland in connection with an alleged plot to kill a Swedish cartoonist who had offended many Muslims.

Irish police said later they had released without charge an American woman, who they didn't identify, and three others arrested in Ireland over an alleged plot to assassinate the cartoonist, Lars Vilks. Three men remained in custody.

Ms Paulin-Ramirez's arrest is one of four developments in the past week that involved Americans in alleged terror plots abroad.

Al Qaida spokesman Adam Gadahn appeared in a video, Sharif Mobley of New Jersey tried to escape his detainment in Yemen, and Colleen LaRose, who allegedly went by the name Jihad Jane to recruit others online to kill Vilks, was named in a federal terror indictment.

Mrs Mott described her daughter as a troubled single mother who had the "mentality of an abused woman" and who, in trying to escape her loneliness, may have spiralled into the depths of Islamic extremism.

Ms Paulin-Ramirez told her family after she left in September that she went to Ireland with her six-year-old son and married an Algerian whom she met online, Mrs Mott said.