A man known for posting an online warning to the creators of "South Park" that they risked death by mocking the Prophet Muhammad was arrested Wednesday and charged with offering himself as a fighter to a Somali terror group linked to al-Qaida.

Zachary A. Chesser, 20, of Oakton, Va., told FBI agents that he twice tried to travel to Somalia to join al-Shabab as a fighter. On the most recent attempt, earlier this month, Chesser brought his infant son with him as he tried to board a flight from New York to Uganda so he would look less suspicious, according to an FBI affidavit.

Chesser was barred from the July 10 flight and told by the Transportation Security Administration that he was on the no-fly list, according to the affidavit.

While Chesser told the FBI that he had intended on July 10 to join al-Shabab, he told them in a July 14 interview that he subsequently changed his mind because of the July 11 bombing in Uganda that killed more than 75 people watching the World Cup, for which al-Shabab claimed responsibility.

But the affidavit from FBI agent Mary Brandt Kinder states that FBI agents first interviewed Chesser in May 2009, and even then he claimed to have moderated his extremist views.

Chesser admitted to the FBI that his intent had been to join al-Shabab, according to the affidavit. He had corresponded with al-Shabab members and expected to undergo a six-week basic training and then serve as a "foreign fighter" with al-Shabab in Mogadishu. According to the affidavit, Chesser expected he would be asked to serve as a propagandist but that he had been willing to fight on the front lines.

His first attempt to join al-Shabab came in November, when he planned to travel with his wife to Kenya and make his way to Somalia, possibly by speedboat. But Chesser wrote in his journal — which he described as a "real-life 'how-to guide' on how to reach the fields of jihad" — that his mother-in-law took his wife's passport and wouldn't give it back, thwarting his plans.

The affidavit also states that Chesser wrote to the radical cleric Anwar al-Awlaki last year, and received two responses. Al-Awlaki is a U.S. citizen now living in Yemen who has recently been designated a terrorist by the U.S. government. Al-Awlaki is believed to have corresponded with several alleged terrorists, including Maj. Nidal Malik Hasan, the U.S. Army psychiatrist accused of killing 13 people in November at Fort Hood, Tex.

Chesser — who also used the name Abu Talhah Al-Amrikee, which designates his American heritage — also told authorities that he used several online profiles to spread terrorist propaganda. One of those sites was al-qimmah, which Chesser described as the official forum for al-Shabab. According to the affidavit, Chesser admitted posting videos and other propaganda on behalf of al-Shabab members.

The affidavit makes only a brief mention of Chesser's warnings to the "South Park" creators, saying that Chesser told authorities his parents were no longer speaking to him because of death threats they received after Chesser posted his warnings.

In April, Chesser posted a warning on the revolutionmuslim.com website that Trey Parker and Matt Stone could face the same fate as a Dutch filmmaker who was killed after making a movie about a woman who rejected Muhammad's teachings. An episode of the show depicted Muhammad in a bear costume.

At the time, Chesser said his online posting was not a threat.

"It's not a threat, but it really is a likely outcome," Chesser told FoxNews.com. "They're going to be basically on a list in the back of the minds of a large number of Muslims. It's just the reality."

The charges against Chesser do not involve the postings about "South Park."

"This case exposes the disturbing reality that extreme radicalization can happen anywhere, including northern Virginia," Neil MacBride, U.S. Attorney for the Eastern District of Virginia, said in a statement.

Chesser's Facebook page indicates he graduated from Oakton High, a public school in Fairfax County, in 2008. he attended classes for one semester in the fall of 2008 at George Mason University, according to GMU spokesman Dan Walsch.

Chesser is expected to make an initial appearance in federal court in Alexandria Thursday on charges of providing material support to a terrorist group. A spokesman for the prosecutors' office said he had no information on whether Chesser currently has a lawyer.