Two Sundays ago, the police say, Lamar J. Platt, a Bronx barber, fatally shot his mother and younger brother, cut up their bodies with a machete and a serrated knife, then packed the pieces into several plastic bags.

That night, Mr. Platt made more than five trips on foot to the Harlem River, the police said yesterday, pushing a shopping cart with the bags along darkened streets, across a highway overpass and through a state park to the swirling riverfront mist. At the water's edge, he dropped the bags in.

Late last week, Mr. Platt showed up for his regular work shift at a barbershop on University Avenue, fellow workers said. He kept to himself, stepping outside frequently to smoke cigarettes, but showed no hint of agitation or worry, they said.

On Sunday, Mr. Platt was taken into custody after the police say he confessed to the horrific killings. But even during his arrest, "he was calm like nothing had happened," said Timothy Waiters, 55, a neighbor who said he had known the Platts for 20 years. "He had that composure through the whole thing."

Yesterday, Mr. Platt, 24, pleaded not guilty at his arraignment before Judge Doris Gonzalez in Bronx Criminal Court on charges including one count of first-degree murder, two counts of second-degree murder and weapons possession. Amy Gallichio, a lawyer who represented Mr. Platt, declined a telephone request for comment on the case. The police said his motives were unclear.

Mr. Platt is accused of killing his mother, Marlene M. Platt, 45, and his brother, Nashan A. Platt, 22, on Nov. 18 in the apartment they shared at 1610 University Avenue in Morris Heights. He spent the next seven days in a world of suspended reality, all but inviting his own apprehension, according to law enforcement officials and acquaintances.

Officers went to the building on Sunday after Ms. Platt's mother and brother, alarmed that she could not be reached by phone, called 911. No one answered the door at the apartment, 1C, and the door was locked.

Lamar Platt showed up at the building, the police said, and as he looked on, the officers forced their way in through a window and found dried streaks of blood in the living room and bedroom. Under questioning by detectives later, the authorities said, he admitted killing his mother and brother, packing the body parts into plastic bags and a shopping cart, and heading out to the river through the nearby Roberto Clemente State Park, crossing the Major Deegan Expressway on the way. Divers searched the river for the bodies yesterday, but had not found anything by nightfall.

Acquaintances described Mr. Platt as an isolated, deeply troubled man who had threatened his mother repeatedly in recent months. "He was always too quiet," said Albert Fortunato, 23, who had known the Platt brothers for more than two years. "A person who is too quiet is planning something."

The police said they were first informed that Marlene and Nashan Platt were missing when the 911 call was placed on Sunday. They said Ms. Platt's mother, Elveda Wright, 63, who had traveled from her home in Washington out of concern for her daughter, confronted Lamar Platt on Sunday at El Magnate, the barbershop and beauty salon where he worked - on the same block where the family lived - and demanded that he turn over a key to the apartment.

Mr. Platt refused, investigators said, but he did not flee and showed no sign of fear that he would be arrested. When the police arrived at the building about 4:30 p.m. and met Ms. Wright, Mr. Platt approached the officers and made contradictory statements before the officers decided to force their way inside.

It was not Mr. Platt's first encounter with the police since his mother and brother had disappeared. On Wednesday, he was arrested on a charge of illegal weapons possession as he stood on a Tremont Avenue sidewalk, near his apartment, after an officer noticed that he was carrying a .38-caliber Smith & Wesson revolver, the police said. He was released on his own recognizance after an arraignment on that charge on Friday before Judge Gilbert Hong of Bronx Criminal Court. Investigators said yesterday that they believed that the same revolver was used in the Nov. 18 shootings.

Neighbors said Ms. Platt had grown increasingly fearful of her son in recent months, and had told them she planned to seek a court order of protection on Nov. 19 - the day after she disappeared. The police said they visited the Platt apartment on Sept. 27, Sept. 28 and Oct. 15 in response to reports of arguments between Ms. Platt and Lamar Platt, but no arrests were made.

Mr. Waiters, the neighbor, said he had received several calls from Ms. Platt about Mr. Platt's threatening behavior. "We, as friends, knew that he was threatening the mother," Mr. Waiters said.

"She was being cautious of him, but she never wanted to put him out," he said.

Ms. Platt, a nurse's aide, had raised both her sons by herself - their fathers live in Jamaica, according to Mr. Waiters - and had continued to care for them, cooking them dinners of oxtail, peas and rice or other Jamaican dishes, and encouraging them to continue living in her home after they had become adults.

Nashan Platt was expected to graduate soon from Lehman College, Mr. Waiters said. Lamar Platt attended Bronx Community College for three years but did not graduate, and had bounced among jobs - most recently at El Magnate.