NEW YORK, FEB. 12 -- The Ethiopian student formally charged today with hijacking a German airliner was an emotionally volatile youth who wanted desperately to join his sister and two brothers in the United States, according to law enforcement officials and the young man's former high school teacher.

In a complaint filed today at the arraignment of Nebiu Zewolde Demeke, 20, FBI officials said he had threatened to kill one flight attendant every five minutes at one point during the 10 1/2 hours that he held 104 people hostage on Lufthansa Flight 592.

Demeke surrendered at John F. Kennedy International Airport Thursday after displaying a weapon and forcing the pilot to divert his Cairo-bound Airbus 310 to New York. Ordered held without bail today by a magistrate in Brooklyn until a hearing Feb. 26, Demeke is charged with aircraft piracy and interfering with a flight crew.

During the brief hearing, Demeke said nothing to indicate his motivation in the alleged hijacking. Obviously nervous, eyes bloodshot, Demeke whispered only "yes" when asked if he understood the charges.

Today, law enforcement officials here and in Germany and an acquaintance of Demeke from his days as a student in Morocco painted a picture of a sad and confused young man seeking to be reunited with his family. Demeke had lived for the last six months in Germany, authorities said.

"He was a very nice kid," said Richard Lee Jackson of the District, who said he taught Demeke English at the American School in Tangiers, Morocco. "But he was very easily distracted. He was very emotional. . . . He would easily cry. I suspect that he desperately wanted to get away from the environment there and really missed his siblings."

Jackson said Demeke is the third of four children raised in Ethiopia. His father, described by authorities as an economist, is a political prisoner in Ethiopia whose family moved to Morocco after he was jailed, Jackson said. "They were trying to escape persecution," he said.

Demeke's older sister, Selamawit, who attends Gettysburg College in Pennsylvania, declined to answer a reporter's questions when contacted by telephone today.

Subsequently, his brothers came to the United States to attend universities, Jackson said. Demeke went to Germany six months ago and applied for political asylum under that country's liberal immigration laws, German officials said.

But after he withdrew his application earlier this month, the German government bought him a ticket on Thursday's Lufthansa flight from Frankfurt to Addis Ababa via Cairo.

German officials were at a loss today to explain how Demeke could board the airliner with a pistol, which U.S. authorities confirmed was a starter's pistol that fires blanks.

But the Germans insisted that security at Frankfurt airport, severely criticized after the 1988 bombing of Pan Am Flight 103 over Lockerbie, Scotland, is not suspect.

"I'm certain Frankfurt is not a weak link," said Klaus Severin, division chief of the Federal Border Police, responsible for airport safety at Frankfurt, which with 90,000 passengers a day is continental Europe's busiest.

According to the FBI complaint in court, Demeke said he placed his weapon under his black hat, which resembled the one worn by the "Indiana Jones" movie character, soon before passing through the security check. At that point, the FBI said, he was examined with a hand-held scanner from the shoulders down.

"The defendant was then asked to remove his hat, which he did, thereby concealing the pistol from the security officer," the complaint said. "The defendant was then permitted to board the aircraft."

Referring to the starter's pistol, Jackson said of Demeke: "He was always a prankster. But this beats everything."

News that Demeke's ticket was provided by the German government provided an ironic twist to the hijacking. Asylum-seekers such as Demeke have been the focus of a debate that has consumed Germany in recent months and been blamed for the outbreak of right-wing violence there.

The government is so eager to lower the number of asylum-seekers in Germany, where 440,000 applied last year, that it offers free tickets to applicants who agree to leave.

Most of the 94 passengers and 10 crew members aboard Flight 592 returned to Frankfurt today and were greeted with flowers, cheers and a welcome from Interior Minister Rudolf Seiters.

"I'm in Frankfurt to make clear that we take this incident extremely seriously," Seiters said. "We'll do everything we can to clear up the open questions."

Gerhard Goebel, captain of the hijacked jetliner, was feted as a hero by the German media for his cool handling of the lengthy ordeal.

Goebel told reporters in Frankfurt that he had to convince the hijacker that the jet needed refueling before crossing the Atlantic. "I showed him on the map where we would sink into the Atlantic if we didn't refuel," he said.

Special correspondents Steve Vogel in Frankfurt and Rachel E. Stassen-Berger in New York contributed to this report.