Story highlights Sulaiman Abu Ghaith gave investigators a 22-page statement, prosecutors say

They say evidence in the case has been sealed

Abu Ghaith is accused of conspiracy to kill U.S. nationals

After 9/11 attacks, he warned that "the storms shall not stop, especially, the airplane storms"

Osama bin Laden's son-in-law, who once sat next to the terror leader and bellicosely threatened the United States, softly answered a judge's questions Friday.

Prosecutors say Sulaiman Abu Ghaith conspired to kill Americans as part of bin Laden's al Qaeda terror network, according to a federal indictment unveiled Thursday.

Through his attorney, the Kuwaiti pleaded not guilty to the charges in a federal courtroom in New York on Friday, nearly a week after his arrest.

A member of bin Laden's inner circle, Abu Ghaith had spent most of his time since the September 11 terrorist attacks in Iran, according to the federal indictment.

Before joining al Qaeda in 2001, he taught high school and preached in his native Kuwait before he was banned from his mosque for using his sermons to attack the government, according to the U.S. government. He also fought in Afghanistan and Bosnia-Herzegovina.

JUST WATCHED Bin Laden's son-in-law held in New York Replay More Videos ... MUST WATCH Bin Laden's son-in-law held in New York 02:20

JUST WATCHED Controversy over Abu Ghaith in NYC court Replay More Videos ... MUST WATCH Controversy over Abu Ghaith in NYC court 02:39

JUST WATCHED Osama bin Laden's son-in-law in court Replay More Videos ... MUST WATCH Osama bin Laden's son-in-law in court 03:16

JUST WATCHED Wyden: U.S. wants tough terror approach Replay More Videos ... MUST WATCH Wyden: U.S. wants tough terror approach 03:47

He spent the months before the September 11 attacks recruiting candidates for training in his father-in-law's terror training camps in Afghanistan, according to the indictment.

After the attacks that destroyed the twin World Trade Center towers in New York and heavily damaged the Pentagon in Washington, Abu Ghaith served as al Qaeda's official spokesman.

In a video made the day after the 9/11 attacks, Abu Ghaith sat next to bin Laden and al Qaeda No. 2 Ayman al-Zawahiri -- an AK-47 behind him resting on a rock wall -- urging Muslims to fight "the Jews, the Christians and America."

He later warned that "the storms shall not stop, especially, the airplane storms," and warned Muslims and foes of the United States not to board aircraft or live in high-rise buildings, according to the indictment.

U.S. authorities have charged that Abu Ghaith supported a major attack on America and urged people at a Kandahar, Afghanistan, guest house to swear an oath of allegiance to bin Laden.

While Attorney General Eric Holder said the arrest sends a message to terror suspects that "there is no corner of the world where you can escape from justice," some analysts said Abu Ghaith is small potatoes in the world of international terrorism.

"This is overblown," CNN National Security Analyst Peter Bergen said . "Though he was Osama bin Laden's son-in-law, Abu Ghaith is far from a big fish in al Qaeda."

U.S. officials arrested him on February 28 in Jordan, according to a spokesman for U.S. Rep. Peter King, chairman of the House Homeland Security Committee. Prosecutors have not said where he was arrested.

According to a report in the Turkish newspaper Hurriyet, Turkish officials detained Abu Ghaith on a tip from the CIA after he had slipped into the country from Iran. After deciding he had not committed a crime in Turkey, Turkish officials decided to send him to Kuwait via Jordan. While Abu Ghaith was in Jordan, U.S. officials captured him, the newspaper said.

The U.S. government has not confirmed the report.

Abu Ghaith -- wearing dark prison-issued clothing, his once dark beard sprinkled with gray -- said little during Friday's hearing. He spoke softly through an interpreter, answering "yes" to several questions from the judge.

According to prosecutors, Abu Ghaith gave investigators a 22-page statement after his arrest, but they didn't reveal any of what the terror suspect allegedly said.

Evidence in the case has been sealed, and prosecutors said they were reviewing classified evidence to see if it would be used in Abu Ghaith's trial.

No trial date has yet been set.

The decision to handle Abu Ghaith's case in civilian court has irked some Republicans, who say the Obama administration is setting a bad precedent and missing an opportunity to get valuable intelligence from someone who had been so close to bin Laden, who died in a 2011 U.S. Navy SEAL raid in Pakistan.

"We should treat enemy combatants like the enemy," said Rep. Mike Rogers, chairman of the House Intelligence Committee. "The U.S. court system is not the appropriate venue."