The mothers of the children discovered the bodies hours after the youngsters had returned from elementary school. It was a scene that would later stun even veteran city police officers.
"There's blood all over my apartment," police dispatch reported a woman saying in a 911 call made around 5:25 p.m. "They've killed my family!"
Police said the call was probably made by a neighbor translating for the mothers, who have difficulty speaking English.
Mayor Martin O'Malley visited the scene last night and urged members of the community to "reach out to one another, support one another."
The mayor described the crime as a "brutal, tragic, unfathomably sad murder of three young children."
As of last night, the man being questioned by detectives was described by police as "a person of interest only" and had not been charged with any crime. He was picked up a couple of blocks from the scene and was described by police as cooperative.
Police said they believe the man had some sort of dispute with the children's relatives, and one of the mothers had pointed investigators to him.
Deputy Police Commissioner Kenneth Blackwell said police are also "looking elsewhere," implying that there may be more suspects. Late last night, police were also trying to determine if there were signs of forced entry.
The fire department's Medic 14 was the first to arrive at the Art Deco-style apartment complex in the 7000 block of Park Heights Ave. late yesterday afternoon.
"You can imagine their feeling when they saw what was inside the apartment," said Fire Department spokesman Kevin Cartwright.
The emergency workers found the children on the floor in two different bedrooms - two 9-year-old sisters in one and a 10-year-old boy in another. The boy had been decapitated.
Police did not immediately identify any of the children last night. While still trying to piece together the family relations, police said they believed there were two families living in the apartment.
The mothers had apparently returned home from food shopping, according to a police source, to find the bodies. The source said the women became frantic and brought a neighbor into the apartment for assistance. The neighbor helped them call 911.
The children were a trio that neighbors said always seemed to be together - playing in the nearby willow tree, asking for the phone number for Chuck E Cheese's.
The first police officer to enter the apartment was overcome by the scene, which Blackwell described as "gruesome" and "something I've not seen before in all the years I've been a part of the agency."
Blackwell said investigators recovered a weapon from outside the apartment. A police source said it was a knife wedged between a wooden fence and a garage in the rear yard of the apartment complex.
No weapons were found inside the apartment, and there was no trace of drugs or drug use, Blackwell said.
Police picked up two priests from St. Ambrose Roman Catholic Church in Central Park Heights to counsel grieving family members. They spent all evening with the two mothers - and the husband of one of them - in the apartment building's leasing office.
"The family is absolutely stunned," said Brother Dennis Klemash.
The 1930s-era Samester Parkway Apartments - which was listed in 1998 on the National Register of Historic Places - is on the southern edge of a neighborhood considered largely Orthodox Jewish.
Neighbors described it as a safe place, one unaccustomed to the swarm of police cars and detectives present yesterday. Police records back that notion: not once in the past six months have officers responded to any unit at the apartment complex, according to a police spokeswoman.
"This is highly unusual for this neighborhood," Blackwell said.
Gregory Carter, who lives in another apartment in the building, said he had seen the children the day before, describing them as "carefree kids."
"Every time I saw them, they was playing cheerfully out in the yard," Carter said. One time, he said, the children came upstairs and knocked on his door asking for a phone number for Chuck E. Cheese's.
Theresa Hopson, 58, who lives nearby, said that she knew the children and would often see them playing together under a willow tree near her apartment window.
"They were beautiful children," she said. "When you saw one, you saw all three. They appeared to be very close to each other. ... It's devastating, absolutely devastating."
Sun staff writers Michael Dresser, Jason Song and Lynn Anderson contributed to this article.