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A US teenager who allegedly fell in love with an Islamist militant online and attempted to join his fight in Syria has been arrested by the FBI on terrorism charges.

Shannon Maureen Conley, a 19-year-old nurse’s aide, was arrested by the FBI on April 8 at Denver International Airport, according to court documents unsealed only today amid an ongoing investigation.

She was trying to board a plane to Frankfurt and then fly on to Adana in Turkey, near the Syrian border, ABC reported.

Conley came across the man — identified only as YM and said to be a 32-year-old Tunisian — online in 2013, the Denver Post reported.

He told her he was fighting for the Islamic State in Iraq and the Levant (ISIL) in Syria, court documents said.

Citing an affidavit, ABC reported that Conley told investigators she planned to meet the man in Syria, marry him and live near the Turkish border.

According to court papers, she received some military training in the US after joining the non-profit US Army Explorers group in September, and “said she intended to use that training to go overseas to wage jihad”, CBS Denver reported.

Conley said she liked the idea of “guerilla warfare because she could do it alone.”

Court documents revealedshe told the Joint Terrorism Task Force that she was a Muslim convert.

Conley was first investigated after a security guard and pastor at the Faith Bible Chapel in Arvada contacted police and the Colorado Bureau of Investigation to report she had been wandering the campus taking notes, the Denver Post also reported..

“If they think I’m a terrorist, I’ll give them something to think I am,” she said in referring to workers at a local church where she had been taking notes.

Her mother Ana Conley told CNN: “It’s a difficult time for us”.

Conley was investigated for eight months before her arrest, the Post reported, citing a federal criminal complaint.

It said federal agents repeatedly met with her and her parents, to dissuade her.

She was charged with conspiracy to provide material support to a designated foreign terrorist organisation. If convicted, she could face up to 15 years in prison, a $A270,489 fine, or both.