The three young sisters found dead last week at their home had been suffocated, and their 3-year-old brother died from skull fractures, police said Monday as they continued to search for the children's missing mother.

Investigators have gone through airline passenger lists, issued U.S. border alerts and dragged a nearby pond in the search for the missing woman, Deysi M. Benitez, 25, but come up empty.

Lt. Thomas Chase said Monday that they would also begin distributing posters with a reference to Benitez's alias, Estela Sedillo, a name discovered on a document among her papers in the family's Frederick home.

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Neighbors and co-workers said late last week that they hadn't seen Benitez for at least 10 days, and that neither the children nor their father, Pedro Rodriguez, had been seen for several days. Rodriguez, 28, was found at the same time as the children; he had hanged himself with nylon rope from a bannister in the family's three-bedroom townhouse.

Police declined to comment on whether any weapon had been found, and they said they did not have any suspects in the children's deaths.

Friends and relatives said Rodriguez and Benitez had emigrated from Sensuntepeque, El Salvador, several years earlier and had struggled with language barriers and financial difficulties while living in the far Washington, D.C., suburbs.

Rodriguez learned March 15 that he would lose his factory job. Benitez worked at a restaurant.

In a telephone interview with The Associated Press from El Salvador last week, Benitez's sister, Angela Benitez, said her sister and Rodriguez were having problems and that he beat her at least once, in December.

"I didn't see it, but she called me and told me that he had left her face a complete mess, that it was a miracle he didn't kill her," Angela Benitez said.

Deysi Benitez asked for a separation, "but he told her he wouldn't allow it," she said.

The children were the 3-year-old boy, Angel Rodriguez, and three girls, Elsa Rodriguez, 9; Vanessa Rodriguez, 4, and Carena Rodriguez, 1. The causes of death given Monday were preliminary determinations from the state Office of the Chief Medical Examiner. The youngest three children were born in the United States.

The parents were legal immigrants, and both had Maryland driver's licenses.