King County prosecutors yesterday charged Ignacio Aguilar Rosas, a 29-year-old transient from California, with first-degree murder in the sexual assault and strangling of a Bellevue mother of four near Bellevue Community College.

In the meantime, detectives are still trying to piece together Rosas' background, but details remain sketchy.

On Monday morning, prosecutors allege, Rosas attacked Cassandra Oliphant, 33, while she was on her way to an appointment.

After a man saw Rosas acting suspiciously, a Bellevue police officer found Rosas lying nude atop Oliphant's partially clothed body, according to prosecution documents.

Rosas remained in King County Jail yesterday with bail set at $2 million. He will be arraigned June 6, and if convicted, could face 20 to 26 years in prison.

In a written statement, King County Prosecuting Attorney Norm Maleng called Oliphant's slaying a "predatory attack" that has "shaken our community, and we grieve for this young woman's family."

Police said they have not found any previous connection between Rosas and Oliphant, a pregnant, single mother with four young daughters.

According to court documents, Rosas and a friend arrived from California on April 27 and lived a "transient lifestyle."

But Rosas also spent some time at an apartment not far from Oliphant's apartment the weekend before the attack. When arrested, he allegedly told police he had been living in a car, the documents say.

At some point, Rosas was introduced to his friend's girlfriend, who later would help police identify a jersey left at the crime scene as belonging to Rosas, according to the court documents.

Oliphant was in the first trimester of her pregnancy, though prosecutors said they will likely not file any charges against Rosas for the death of the fetus, said Dan Donohoe, prosecutors' spokesman. Generally, an unborn child must be able to survive outside the womb in order for someone to be charged with its death, Donohoe said.

But Rosas could face other charges in the attack, depending on the results of tests on "trace evidence" that police said they found at the crime scene.

Oliphant died on a makeshift trail near her Bellevue apartment complex, where she was known as a kind neighbor. She had trouble finding work, but just before her death she had told friends that she had a line on a new job.

Bellevue school officials have set up a donation fund for Oliphant's four daughters, ages 4 to 12, who are expected to be cared for by family members. Donations can be made at any Bank of America branch to the Friends of the Oliphant Family fund.

Ashley Bach: 206-464-2567 or abach@seattletimes.com