One of the family's neighbors, Keith Lamirande, 41, said he didn't see the family much.
Property records, which valued the home near Indian Spring Village at more than $450,000 in 2007, indicate that it is owned by Amy Castillo.
At one point yesterday, two men were seen inside, opening the door for six people just arriving.
Maria and Octave Habesch, who live on the street, knew the family and said they had been neighbors for a while.
Maria Habesch, 79, said she last saw the children Friday, playing in front of the house with other kids. She hadn't seen Mark Castillo in two years, she said, but described him as very friendly.
"He was always very polite to us, always acknowledging me," she said.
"I can't believe it - the kids," she said. "They were such a lovely family. ... Oh, beautiful kids."
Yesterday afternoon at the Marriott, city police officers, hotel security and red-uniformed bellhops stood in the lobby to prevent nonguests from entering the building. A homicide detective interviewed one man in a chair, while people checked in at the front desk.
Top police commanders, including Bealefeld and Deputy Police Commissioner Anthony Barksdale, came to the scene. Detectives were getting a search warrant to collect evidence from the room Mark Castillo checked into Saturday.
Outside, police cruisers lined West Lombard and Eutaw streets, where the hotel is located, along with a firetruck and two ambulances. Employees from the neighboring Holiday Inn, as well as curious passers-by, stood on the sidewalk, speculating as to what might have happened.
Larry Laguda, a cabdriver, said he was standing in front of the hotel when police cars arrived in large numbers.
Laguda said he went to the hotel door to find out what was going on. In the six or so years he's driven in the area, he said, he's never seen "something of this magnitude."
He was shocked to hear that three children were killed, and shook his head in disbelief.
Several guests who had come to the hotel yesterday for a brunch with radio station WSMJ shared his reaction.
Hotel officials declined to comment, citing the police investigation.
"We cannot divulge any information at this time," said Cornel Jones, director of security.
Over the past few years, there have been several incidents in Maryland involving fathers or other relatives accused of killing young children.
Last month, a West Baltimore man said "demons" made him throw his 3-year-old son off the Key Bridge. Stephen Todd Nelson, 37, was charged with first-degree murder. He and the child's mother, Natisha Johnson, had battled in court over custody and visitation issues.
At Thanksgiving, David Peter Brockdorff, 40, killed his former wife, her three children and himself with a .22-caliber rifle. The bodies were found in a Montgomery County park. The couple had divorced.
In March last year, four children were found dead in their Frederick County house. Their father, Pedro Rodriguez, 28, was found hanged in the foyer.
In May 2004, three Mexican children were killed in Northwest Baltimore. Two older family members were convicted of the killings and are serving two consecutive life sentences without parole.
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Sun reporters Richard Irwin and Tom Pelton contributed to this article.