An Iowa man suspected of brutally slaying seven people, including a mother and her five children, will appear in court Saturday on a theft charge and is expected to face formal charges for the deaths next week.

Adam Matthew Moss, 23, identified as the slain woman's boyfriend, was taken into custody by Sioux City police without a struggle on Friday, Police Chief Joe Frisbie said.

He said Moss has not yet been formally charged, but he is a suspect in the slayings of 31-year-old Leticia Aguilar and her five children: Claudia, 12, Zach, 11, Larry, 9, Lisa, 7, and Michael, 6. Moss is also a suspect in the slaying of 58-year-old Ronald Earl Fish, whose body was found in a home across town.

The bodies of the family were discovered Thursday night by a baby sitter and appeared to have been there for a few days, authorities said.

Frisbie said stress evaluation would be offered to the officers who responded to the scene, which he described as "brutal." He said the mass-murder was "probably one of the worst in state history."

Baby sitter Donna Stabile, who found the family's bodies, said the sight was gruesome.

"I saw two kids lying face down, and there was a lot of blood. I could barely see them because the place was such a mess."

Frisbie said Moss and the woman had both worked for Smurfit-Stone Container Co. The company issued a statement Friday saying Moss had quit his job only recently.

Stabile said the man living with the mother had lost his job at Smurfit-Stone about three weeks ago.

"I didn't know him that well," Stabile said. "He was always polite. The kids liked him. She liked him. But I had my suspicions. He was a nice enough guy, but I didn't see them together a lot."

Before the suspect was caught, Frisbie said he believed Moss was armed and still in the area.

"We have been about a half-hour behind him all night," he said. "This individual can make a weapon out of anything, I'll tell you that."

Frisbie said the names of the woman and her children, found in a two-story home in a low-income neighborhood, would not be released until relatives had been notified.

The mother's and children's bodies were found in an older two-story home in a low-income neighborhood. All six bodies were removed from the house about 3:30 a.m. CDT Friday.

One neighbor, Sharon Morrison, said the last time she had seen the woman was Monday when she walked outside to check the home's air conditioner.

Lynn Pretends Eagle, another neighbor, called the deaths "absolutely devastating."

"There's no words to describe it," she said. "Five children got killed. Five kids and their mother."

Mark Stroman said he was a friend of Moss and had worked for Fish, the owner of a tire company.

"The last time I'd seen [Moss], he said he just got out of prison for drugs, assault and criminal mischief. He said he was living with a girl and he seemed like he was getting his life straight," Stroman said.

Earlier in the day, police issued an arrest warrant for Moss for the theft of a car belonging to Fish that had been found and impounded.

Fish was found by a co-worker who became concerned when he didn't show up for work, police said.

Moss was placed on one year probation in 1995 for assault with injury, a serious misdemeanor. The day he finished probation, he was given a 5-year suspended sentence and two years probation for third-degree burglary and second-degree theft, both felonies. He was discharged from his probation Jan. 8, 1998.

Frisbee said he and the community were relieved that Moss was in custody.

He said it was a very dangerous situation for more than 12 hours, with police playing "cat-and-mouse games" with Moss.

Sioux City, on the border of Nebraska and South Dakota, has about 84,000 residents.

Mayor Marty Dougherty told The Des Moines Register that the town averages two homicides at most per year. "This just sounds really, really awful," he said.

The Associated Press contributed to this report.