Manassas Police Detective Edwin Rivera was grabbing a takeout order of chicken tacos late Monday afternoon at the Georgetown South shopping center when he heard a woman's scream coming from a neighboring general store.

He went to investigate and saw a man in a black sweat suit bursting out of the Rosita Shop and run away. Rivera, who has spent a decade in law enforcement, immediately thought the man had stolen something.

The case turned out to be much more serious than a misdemeanor. After Rivera chased the suspect, he turned back and saw a bloodied woman staggering out of the shop. He called for backup. The woman, Rosa Isabel Umana, 30, later died of a stab wound to the heart. Authorities said she was four months pregnant.

If Rivera hadn't been at the shopping center grabbing an early dinner during his shift, it would have been difficult to find suspects or make an arrest, and the case likely "would have been an open homicide," he said. Police eventually tracked a suspect down at a nearby home they said he had broken into. Jorge Vazquez-Hernandez, 21, was arrested and charged with first-degree murder in the stabbing of Umana, who was a clerk at the Rosita Shop at the shopping center in the 9600 block of Grant Avenue. Vazquez-Hernandez, of no fixed address, remains held without bond at the Prince William-Manassas Regional Adult Detention Center and faces a court hearing Dec. 13.

Police said Umana, of the 8500 block of Yoder Street in Manassas, died at the scene. She was also stabbed in the torso and the face, said Sgt. Tim Neumann, a Manassas police spokesman.

Commonwealth's Attorney Paul B. Ebert said that more charges could be filed, especially if it is determined that Umana's fetus was viable. In Virginia, someone who kills a pregnant woman with the intention of killing the fetus can be charged with capital murder, which can carry the death penalty.

Rivera said that no money or jewelry was stolen, leading him to think the slaying could have risen from a dispute between Vazquez-Hernandez and Umana.

Police are still trying to determine a motive for the attack, and whether Vazquez-Hernandez and Umana knew each other and, if so, to what extent.

"We just don't know the relationship between the two, or if it was a robbery gone bad, or if he just went in there and tried to buy something and didn't agree with the price," Neumann said.

Several officers joined the search for Vazquez-Hernandez, who had run a relatively short distance from the shopping center to a home on the 9200 block of Taney Road. A resident in the area told an officer that someone had run into a shed that connects to one of the townhouses in the neighborhood, Rivera said.

Rivera said officers entered the home through the front door and discovered Vazquez-Hernandez at the top of a stairwell. Police ordered him to come down, and he was arrested without incident.

Rivera said that Vazquez-Hernandez was not carrying a weapon when he was arrested and that he had blood on his fingers and clothing. Neumann said that a knife was recovered at the shopping center and that police are still trying to confirm whether it was used in the attack.

Juan Chicas, owner of the Chicken Place II, where Rivera had ordered his takeout meal, said the scene outside the general store was somber.

"I was pretty shocked," Chicas said. "She was dead when the officer came and went to check her pulse. He just got up, left and shook his head."

In a brief interview at Umana's home on Yoder Street, Jose Orellana, 26, who said he had been married to the victim briefly, said he did not know Vazquez-Hernandez.

On Tuesday, as he flipped through photographs of Umana smiling and standing inside the store, all he could say was that he felt very sad.