In this undated picture posted on the Facebook page of a militant group, members of the Syrian rebel group Ahrar al-Sham exercise in a training camp at an unknown place in Syria. (AP)

Federal authorities have arrested a naturalized U.S. citizen who they say was part of a conspiracy to provide firearm scopes, tactical vests and other supplies to a Syrian rebel group that “frequently fights alongside” al-Qaeda’s affiliate in that country.

Amin al-Baroudi, 50, who also goes by Abu al-Jud, was charged in an indictment unsealed last week with violating U.S. sanctions against Syria and conspiring to defraud the United States. Court records show that Baroudi was formally served with his charges on Thursday at Dulles International Airport. An indictment had been filed against him secretly in federal district court in Alexandria in April.

A burly man with a salt-and-pepper beard, Baroudi said nothing Monday at a brief court appearance at which a federal magistrate judge ordered him detained pending further legal proceedings. His attorney, Anthony Capozzolo, said in court that Baroudi had been to Saudi Arabia and suffered a heart attack there but offered no other details about the case or Baroudi’s background.

Capozzolo said in court that he was working to prepare materials that would bolster Baroudi’s case to be released on bail. He declined to comment after the hearing.

Baroudi’s arrest comes amid heightened fears of terrorism in the United States stemming from recent terrorist-inspired or directed mass killings in San Bernardino, Calif., and Paris. Significantly, though, Baroudi is not charged with lending support to terrorists, and the particular group he is accused of supplying with weaponry, Ahrar al-Sham, has disputed that it has links to al-Qaeda or espouses al-Qaeda’s ideology.

Labib Al Nahhas, Ahrar al-Sham’s head of foreign political relations, wrote on The Washington Post website in July that the group, which is among others trying to remove Syrian President Bashar al-Assad from power, believes “in a moderate future for Syria that preserves the state and institutes reforms that benefit all Syrians.”

“We consider ourselves a mainstream Sunni Islamic group that is led by Syrians and fights for Syrians,” he wrote.

In Baroudi’s indictment, federal prosecutors characterized the group as one that “frequently fights alongside Jabhat al-Nusra, which has been designated by the United States as a foreign rerrorist organization and operates as al-Qaeda’s official branch in Syria.” They alleged that Ahrar al-Sham’s “stated goal is to overthrow the Assad government and install an Islamic state in Syria.” The United States also opposes Assad.

Baroudi, who is a Syrian-born, naturalized U.S. citizen, had been living in Irvine, Calif., though the manner in which he was indicted suggests he was brought to Dulles Airport specifically from overseas so that he could be charged in Alexandria. A woman who identified herself as the daughter of a 50-year-old Amin al-Baroudi from Irvine, Calif, declined to comment.

The indictment alleges that Baroudi and others bought supplies from companies in the United States and traveled with the goods on commercial flights to Turkey, where someone would slip across the border into Syria. Baroudi, according to the indictment, did not have a license to do so and was thus violating a broad prohibition on exporting goods to Syria.

According to the indictment, Baroudi bought scopes, laser sights, tactical vests and other equipment that might be useful in military endeavors, often using common websites such as eBay and Amazon. He talked with others of having purchased tens of thousands of dollars worth of goods, though the exact amount of money he and others made is unclear.

In one instance, when Baroudi made an overseas trip in early 2013, he flew out of Los Angeles with 14 checked bags weighing 619 pounds and returned with just two. Prosecutors alleged the conspiracy lasted from late 2011 into 2013.

Prosecutors alleged that Baroudi knew he had not obtained an export license and went ahead with his plan anyway, and they hinted, too, that he was at least aware of questions about Ahrar al-Sham’s goals.

In August 2012, according to the indictment, someone warned Baroudi that Ahrar al-Sham was dangerous and, unlike other rebel groups, wanted to establish an Islamic state in Syria. Baroudi responded that he had a good relationship with the group, according to the indictment.

In December 2012, according to the indictment, the same person told Baroudi that Ahrar al-Sham was “Qaeda less 25 percent.” Baroudi’s precise response to that assertion is unclear, though prosecutors alleged that Baroudi said he intended to bring more than $30,000 in equipment to Syria and train people there to use it.