CHICAGO (AP) -- Police searched Tuesday for a 28-year-old convicted felon they say may be connected to a string of eight recent killings in Illinois and Missouri.

Nicholas T. Sheley was last seen on the two states' border near St. Louis and should be considered armed and dangerous, police said.

He has not been charged in any of the slayings, but has been named a person of interest in at least five killings in western Illinois and may have a connection to three others there and in Missouri, according to Illinois State Police Sgt. Thomas Burek.

Seven of the eight were found dead on Monday and police were looking for stolen vehicles in at least two of the cases.

The string of murders began with the beating death of 93-year-old Russell Reed, a Sterling man whose body was found stuffed in a trunk on Thursday. Sheley also is from Sterling, a small town about 100 miles west of Chicago.

On Monday, the bodies of two men, a woman and a child were discovered in an apartment in nearby Rock Falls after a well-being check. Police wouldn't say who made the request for the check, but they believe one of the victims is connected to Reed.

Burek said Sheley is an acquaintance of one of those found inside the apartment.

Also Monday, the body of an older man was found near a trash bin behind a grocery store in Galesburg, a community about 60 miles southwest of Rock Falls.

Police Sgt. James Martinson said investigators were working to determine if the body is that of a 65-year-old Galesburg man who was reported missing Monday. The man's car was stolen and later found in Missouri, Martinson said, but he did not say exactly where. An autopsy was planned for Tuesday.

Meanwhile, the bodies of a man and woman also were found Monday behind a gas station in the St. Louis suburb of Festus, Mo., about 250 miles further south of Galesburg. Investigators were looking for the couple's pickup truck.

Festus is about 50 miles south of Collinsville, Ill., where police say Sheley had been seen Monday night.

On Tuesday morning, police in tactical gear searched a Collinsville apartment building for a person of interest the Festus killings, but "were unable to locate the guy," Collinsville police Sgt. Rich Wittenauer said.

Investigators said Sheley remained at large Tuesday and "should be considered armed and dangerous." Public records show he has a lengthy criminal record, including multiple convictions for robbery, drug and weapons charges.

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Associated Press Writer Jim Suhr contributed to this report from St. Louis.