RIVERSIDE, Calif. -- A man suspected of being the ringleader of a plot to kill Americans and bomb U.S. military bases overseas has been returned from Afghanistan, authorities said Wednesday.

American Sohiel Omar Kabir, 34, made his first court appearance Tuesday in the U.S. after he was captured by U.S. special forces in Afghanistan last month, said his attorney, Deputy Federal Public Defender Jeffrey Aaron.

Kabir has not yet entered a plea after being charged with conspiracy to provide material support to terrorists. He is being held without bond and scheduled to appear in court Dec. 11.

Meanwhile, Ralph Deleon and Arifeen Gojail, both 21, and Miguel Santana Vidriales, 23, pleaded not guilty Wednesday after being indicted on the same charge.

If convicted, each of the four defendants could face up to 15 years in prison.

Deleon, Vidriales and Gojali were arrested as they waited to board a plane headed for Istanbul on their way to Afghanistan to meet with Kabir, authorities said.

In video calls from Afghanistan, Kabir told the trio he would arrange meetings with terrorists, investigators said.

The group prepared for their trip to the Middle East by simulating combat with paintball rifles and concocting cover stories, court documents state.

Authorities don't believe there were any plans for an attack in the U.S., but Deleon and Vidriales told a confidential FBI informant they would consider American jihad, according to an FBI affidavit.

The case against Kabir is based on hearsay statements from co-defendants and an FBI informant, Aaron said, adding that federal prosecutors have turned over few documents since the arrest.

He added that his client suffered physical injuries to his face and head when he was captured in Kabul, where he was staying with family members.

"He wasn't hospitalized but he probably should have been," Aaron said.

FBI spokesman Laura Eimiller said Kabir suffered "combat-related injuries" during his capture. The injuries were treated by American medical personnel and he was cleared to be taken back to the U.S.

Kabir, a naturalized U.S. citizen from Afghanistan, served in the Air Force from 2000 to 2001 and introduced Deleon and Santana to radical Islamic doctrine, investigators said. Gojali, also a U.S. citizen, was recruited in late September.

Experts who study homegrown terrorism said the case highlights the susceptibility to radicalization of new converts to Islam, particularly among the young.

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