Michael Devlin was already charged with kidnapping Ben Ownby, 13, in January in Franklin County and with kidnapping Shawn Hornbeck, then 11, in 2002 in Washington County.
He also was charged with several counts of forcible sodomy in St. Louis County, where Devlin allegedly kept the boys, and with federal counts of pornography and transporting a boy across state lines.
Washington County prosecutor John Rupp filed the additional charges Monday against Devlin, accusing the 41-year-old of attempted murder, kidnapping, armed criminal action, three counts of forcible sodomy and one count of attempted forcible sodomy.
The new charges allege that Devlin tried to suffocate "SH" after kidnapping him. The papers only give the initials, which match Shawn's name.
Rupp declined to discuss details of the charges. He said the crimes occurred through early November 2002 in Washington County but would not say if Devlin was keeping Shawn in the county, instead of at his apartment in St. Louis County. Shawn disappeared in October 2002.
A spokesman for Devlin's attorneys declined immediate comment.
Shawn and Ben were found Jan. 12 at Devlin's apartment in Kirkwood. Ben had been kidnapped four days earlier after getting off a school bus near his home in the Franklin County town of Beaufort. Shawn, now 15, had been riding his bike in the Washington County town of Richwoods when he disappeared.
Investigators have previously confirmed they were checking for potential connections between Devlin and at least four cases in Missouri, one in Illinois and one in Minnesota, cases that date to the late 1980s. Until his arrest in January, Devlin had no criminal record.
While it is The Associated Press' policy not to identify alleged victims of sexual abuse in most cases, the story of Shawn and Ben has been widely publicized and their names are now well-known. Also, their families have gone public, conducting several media interviews.