Of the four crashes, the one of Flight 175 seemed, in some ways, the most chillingly deliberate.

While the attention of the world was riveted on the already damaged north tower of the World Trade Center, it was United's Flight 175 that plowed into the south tower with a homicidal theatricality its planners must have known would be broadcast everywhere.

As details began to emerge yesterday, the story of Flight 175's 50-minute final flight terrified the friends and family members left behind as much for what they did not know as for what they did.

''There is heart-wrenching empathy for what we assume they went through,'' said Andrew Freedman, a friend, speaking for the family of Ruth Clifford McCourt of New London, Conn., who died on Flight 175 along with her 4-year-old daughter, Juliana.

At Logan International Airport in Boston, Flight 175 took off on time with just 56 passengers in the big Boeing 767. It left the gate at 7:58. Its wheels were in the air by 8:15. Victor J. Saracini, 50, an experienced pilot who had been a Navy flier, was at the controls.

Between takeoff and 9:03, the moment of impact, a few things are known.

The flight was on course heading southwest toward Los Angeles until 8:47, when, west of the George Washington Bridge over New Jersey, the plane made a sharp left turn. Twelve minutes later, it made another sharp left to settle on a course leading directly to the south tower.

At some point, men armed with knives stabbed flight attendants, a cell phone caller from the plane said in several brief calls to his father in Connecticut. Relatives of the caller, Peter Hanson, 32, a software executive, told reporters that the hijackers seemed to be trying to force the crew to open the cockpit doors.

''The plane,'' Mr. Hanson said, ''is going down.'' He was traveling with his wife and their 2-year-old daughter.

Flight 175, like each of the planes that were used as weapons on Tuesday, included people traveling for many reasons. Mrs. McCourt and Juliana were heading to a spiritual center in California and, perhaps, to Disneyland.

There were businessmen and tourists. One passenger, Al Marchand, was an off-duty flight attendant who was a retired New Mexico police lieutenant.

Two scouts from the Los Angeles Kings hockey team were passengers, Mark Bavis, 31, who had played on Boston University's team, and Garnet Bailey, known as Ace, a former National Hockey League player who had befriended Wayne Gretzky on the Edmonton Oilers. The hockey scouts were headed to the Kings' training camp in El Segundo, Calif.

Some of the passengers had ended up on the flight by chance. Others had planned carefully. But, still, in life as it was until Tuesday anyway, details of airlines and flight numbers were treated as incidental, not significant enough for loved ones to dwell upon in advance.

Yesterday, Alasdair Drysdale, the chairman of the geography department at the University of New Hampshire, described how, late on the day of the crashes, he had talked with the wife of his colleague Robert LeBlanc, who recently retired as a professor at the university and had been on his way to an academic conference in Los Angeles.

Mrs. LeBlanc, Mr. Drysdale said, was unsure of her husband's precise travel arrangements and still hoped he had been on some other plane. But there it was in Professor LeBlanc's daily diary at the office. ''It said: '8 o'clock, leave Boston,' '' Mr. Drysdale said.

Among the planners on the flight was Alona Avraham, 30, an Israeli who had come to the United States for the first time.

Ms. Avraham had planned every detail, including the flight she would take after a stay with a friend in Boston as she headed to the home of a cousin who owned an auto repair shop in Los Angeles.

''In Israel, it is bombing and shooting,'' said the cousin, Danny Raymond, ''and her dream was just to come to the U.S., just for a visit. For a week or two and then go back.''

On Monday she had called home to Ashdod, near Tel Aviv, her mother, Miriam, said in a telephone interview from Israel.

Like so many of the other passengers, her final words to loved ones were ordinary. '''Everything's fine, Mother. I'm packing,' and that's all,'' Mrs. Avraham said.

Of course, somewhere in those rows as the plane lifted off were passengers who knew this would be anything but an ordinary flight. So far, it is not certain who they were.

The Boston Herald reported yesterday that two brothers with passports from the United Arab Emirates had been on the plane. One, the newspaper said, was a trained pilot. Massachusetts investigators, The Herald said, had concluded that the two men had boarded Flight 175, perhaps after a drive from Portland, Me., and before that Canada.

Reuters reported yesterday that the men were Saudi citizens with international driver's licenses issued in the United Arab Emirates in June.

Mr. Raymond, the repair shop owner, said he had left for work in the morning promising his three children, ages 15, 12 and 9, that he would bring their cousin from Israel when he returned home.

When he came back without her, he said, the children did not seem to understand. ''They were saying, 'Where is Alona?' '' he said. ''I couldn't even wait, I just started to cry. We were all hugging each other.''

By then, the video pictures of Flight 175 hitting the south tower had been seen countless times around the world.

Map of the east coast shows the route of United Flight 175.