The plane, a 50-seat regional aircraft that was less than a third full when it took off from La Guardia Airport, had been climbing through the early-morning sky for about 25 minutes. A 17-year-old passenger in a whitish sweater took out something he had carried onboard, and strapped it onto his wrist and his head.

To some people in New York, that is a relatively common sight: an observant Jew beginning the ritual of morning prayer. But to at least one person on US Airways Express Flight 3079 on Thursday — the flight attendant — it looked ominous, as if the young man were wrapping himself in cables or wires.

And in a time when in-flight thinking is colored by the brutal knowledge that passengers have hidden bombs in underwear or shoes, she told the officers in the cockpit.

The pilot decided to divert the Kentucky-bound plane to Philadelphia. In less than 30 minutes it was on the ground, police officers were swarming through the passenger cabin, and the Transportation Security Administration was using terms like “disruptive passenger” and “suspicious passenger” to describe the boy.

An hour or so after that, Lt. Frank Vanore, a spokesman for the Philadelphia police, had another explanation.

“It was unfamiliarity that caused this,” he said.

He said the flight crew had never seen tefillin, small leather boxes attached to leather straps that observant Jews wear during morning prayers. The flight crew “didn’t understand what it was,” he said, and the pilot “erred on the side of caution and decided to radio that in and to divert the flight.”

The boy’s grandmother, Frances Winchell, said it was just one of those things. “It’s true that we in America are very, very skittish,” she said at the airport in Louisville, where she had been waiting to meet the boy and his 13-year-old sister, who was also on Flight 3079. Mrs. Winchell said she hoped people would learn about the rituals and not be fearful.

The young man and his sister, whose names were not released, are from White Plains, the authorities said. Rabbi Shmuel Greenberg of Young Israel of White Plains said that they were members of his congregation and that the young man was “a good boy, bright, intelligent, as docile as you can imagine.”

“He didn’t think of the ramifications, I guess,” Rabbi Greenberg said. “You can’t expect the whole world to know what this ritual is all about.”

He said men in his congregation recited morning prayers “if at all possible within four hours after sunrise.”

“Nobody would have assumed it would create panic,” he said, “but in today’s environment, I guess everything creates panic.”

In fact, Lieutenant Vanore said the other passengers did not know exactly why the plane went to Philadelphia until it was on the ground and police officers and federal agents began checking for explosives. They found none. The young man was “was completely cooperative” and explained the prayer ritual, Lieutenant Vanore said.

He explained that the scare began when the young man was “in the process of praying.” The flight attendant noticed the tefillin and asked what he was doing.

The young man’s response was straightforward, Lieutenant Vanore said: “He gave the explanation he was in prayer.”

But the flight attendant was concerned about the tefillin. She called the cockpit and “described it as best as she’d seen it,” Lieutenant Vanore said, “and there was an item wrapped around his head, straps or wires.” “The straps did appear to be cables or wires to her,” he said. “To the naked eye looking at it, it looked like that. She said it had wires running from it and going up to his fingers. When they notified the pilot of that, he had to follow his protocol. It’s hard to Monday-morning-quarterback it.”

The young man and his sister went on to Louisville from Philadelphia. He said nothing as he walked through the terminal in Louisville in mid-afternoon.

Reached at her home on Thursday night, Ms. Winchell would not put her grandson or granddaughter on the telephone, but she allowed a reporter to ask questions on a speakerphone while they were present. Ms. Winchell said that police officers who boarded the plane pointed their guns “at the passengers as a whole” and at her grandson “a little bit.” She said her grandson and granddaughter were then handcuffed for a few minutes.

“The handcuffs were only for a short period of time,” she said.

A spokesman for the Philadelphia police could not be reached for comment after the interview with Ms. Winchell.

Some observant Jews said they were not surprised that the ritual had attracted attention — or that people on the plane would have been unfamiliar with it. “When they see a passenger strapping yourself,” said Isaac Abraham, a Satmar who lives in Williamsburg, Brooklyn, and campaigned for the Democratic nomination for a City Council seat last year, “you might as well strap yourself with hand grenades. They have no idea.”

“He probably just figured, ‘I have nothing else to do on the plane, I might as well use this time to pray,’ ” he added. “Other people read. They watch a movie. He figured, ‘Let me grab the time.’ But the obvious reality of it is that when we see people carrying explosive material in their shoes and their pants and I am the passenger next to him and see someone strapping, I would panic too.”

Rabbi Greenberg, the boy’s rabbi, had some advice for future flights.

“I would suggest, pray on the plane and put the tefillin on later on,” he said. “Pray, and fulfill the ritual later.”