Two Mexican immigrants who joined relatives grieving for three slain children Thursday night were arrested at the crime scene and charged yesterday with the killings, police said.

Baltimore police said last night that they still did not know of a motive for the decapitations - killings so horrific that a Northwest Baltimore neighborhood remained shaken yesterday.

But among the theories being explored was revenge over an unpaid debt for transportation into the United States as undocumented immigrants.

Police say the two charged were an uncle and a cousin of the victims. A fillet knife with at least a 10-inch blade was used to decapitate one child and partially behead the other two.

The suspects, both from Baltimore County, were identified shortly after the killings by a woman at the apartment complex in the 7000 block of Park Heights Ave. where the children were killed, a police source said. The woman told city police that she had seen two suspicious men, dressed in black, canvassing the Samester apartment complex this week.

She said she would recognize the men, and then she saw them at the crime scene.

Officers arrested Adan Espinoza Canela, 17, a butcher at a Baltimore slaughterhouse, and Policarpio Espinoza, 22, who was involved in the family business of selling food from a truck, said Detective Irv Bradley.

Police officials described the men as "emotionless" at their arrest, just hours after the Thursday afternoon killings. Both were charged with three counts of first-degree murder and one count of possession of a deadly weapon, said a spokesman for the state's attorney's office. They are being held at Central Booking and expected to have a bail review hearing Tuesday.

"This is an act of someone who obviously has no conscience," Deputy Police Commissioner Kenneth Blackwell said at a news conference yesterday.

Police sources said one motive that detectives are exploring is whether the killing is related to a debt the family owed for being brought across the border. The family is undocumented, officials at the Mexican Embassy in Washington said.

Federal immigration officials assisted at the crime scene.

The dead children were identified yesterday by police as Alexis Espejo Quesada, a 10-year-old boy; Lucero Quesada, a 9-year-old girl; and Ricardo A. Espinoza, a 9-year-old boy. But city officials acknowledged yesterday that they remained unsure of the spellings.

Ricardo and Lucero are siblings, and Alexis is their cousin, officials said.

Ricardo and Lucero's family has been in the country about seven years, the last three in Baltimore, officials and neighbors said. Alexis' family arrived about five months ago, said embassy spokesman Miguel Monterrubia.

The mother of one of the victims also has a 2-year-old daughter, who was with a babysitter, Bradley said.

Police had previously misidentified the children as two girls and a boy. They were in third and fourth grades at Cross County Elementary School.

The day after their deaths, the children's classmates were comforted by grief counselors. Their mothers traveled to the Mexican consulate in Washington, preparing to ship the bodies back to their native Veracruz, Mexico, for a funeral.

As many as 10 people may have lived in the apartment where the children were killed, said Antonio Williams, the chief who oversees the detective division.

On Thursday, the children arrived home from school about 3:30 p.m. Neighbors said they typically go outside to play among the trees and cicadas in the complex's grassy courtyard.

On Wednesday, they had collected cicadas in a pickle jar with holes on the top, said Tyler Ramseur, a neighbor who frequently saw them playing in the courtyard.

Another neighbor, Tamiko MacDonald, said she didn't see them outside Thursday.

According to people familiar with the case, the children apparently let their attackers into the apartment because they knew them. The attackers used the knife to behead the children, according to police.

About 5:20 p.m., the mothers returned home and discovered the children. Two children lay in one bedroom, one in another.

A neighbor helped the women, who speak little English, to call 911. He reported that a woman had discovered her family dead.

The scene was so grisly that the first officer on the scene was overcome and had to walk away. A priest from St. Ambrose Roman Catholic Church in Central Park Heights cried as he gave the children the last rites.

At the time, the mothers pointed police to a man whom officers detained and described as a "person of interest." Police spokesman Matt Jablow said that person was released without being charged.

Shortly after the killings, police found the knife, which apparently has a clublike handle, wedged between a wooden fence and a garage in the rear of the complex.

Police removed large amounts of evidence from the first-floor apartment, Williams said.

After being taken to headquarters, one of the suspects incriminated the other about 2 a.m., a police source said. Detectives consulted with the state's attorney's office and decided to charge them. Williams described Canela and Espinoza as the "main suspects" but did not rule out more arrests.

Yesterday, detectives executed search warrants on a Baltimore County house, just northwest of the city line, where the Espinoza and Canela lived.

They resided on leafy Bedford Road with a number of relatives - including three or four children, police and neighbors said.

Property records show that Victor Espinoza and Carmelo Juarez - relatives of the two men, according to police - bought the house in October 2002 for $134,000. Neighbors said it seemed as if 10 people might be living in the home.

Enez and Dilworth Sladden, who regularly visit their daughter next door to the Espinoza family, said the older couple seemed to work hard, leaving the home early each morning in the camperlike food truck.

But a couple of men, who looked to be in their 20s, who seemed to hang out all day, drinking and smoking, Dilworth Sladden, 70, said. Sladden said he didn't know the family well, in part because they did not seem to speak English, and he does not speak Spanish.

"I'd say, 'Que pasa?' and they'd wave," Sladden said.

As police searched the home, city housing officials said they were helping the victims' family settle into an apartment in Baltimore County.

It remained quiet after school in the courtyard at the apartments where the children had lived. Exceptions were the cicadas and the reporters, including many from Spanish-language networks and publications.

Starr Davis, an 11-year-old girl who identified herself as Lucero's best friend, said the two girls liked to play house.

"She always called me mommy," Starr said.

Starr's mother, Nancy Davis, said the kids were "always dressed to go somewhere." Davis said she would frequently drive the family to church.

"It's so hurtful," Davis said. "You see them one day, and they're gone the next. You're not supposed to go like this."

Sun staff writers Richard Irwin, Kimberly A.C. Wilson, Michael Dresser and Frank Langfitt contributed to this article..