Authorities didn't say why they think the two cases are linked, but they noted Sigg will be charged in both crimes. His first court appearance is set for 8 a.m. Thursday.
Police said they notified the Ridgeway family of the arrest Wednesday morning.
"We hope and pray that this arrest brings them some measure of closure in dealing with this enormous loss that they've suffered," Police Chief Lee Birk said.
The arrest also brought relief to the community, which has been on edge as authorities searched for Jessica's killer. More parents have been waiting with their children at bus stops, and high school students have volunteered to walk younger children to school to keep them safe.
"Every parent in every Colorado community will rest a little easier tonight," said Gov. John Hickenlooper, who has a 10-year-old son. "While we still mourn the death of Jessica Ridgeway, we are relieved an arrest has been made and the pursuit of justice can continue."
Jessica was walking down a quiet street in her modest Westminster neighborhood when she was last seen alive. Her school backpack was found three days later in Superior, another Denver suburb about seven miles northwest of her home.
After her disappearance, more than 1,000 officers and 10 agencies, including the FBI, investigated the case, following up on more than 4,000 leads.
Authorities had long said Jessica's killer could be someone from the community. But early on, they didn't seem to have any particular suspect in mind. They asked residents to be on the lookout for anything suspicious from their bosses, friends and family members, watching for things like leaving home unexpectedly, missing appointments or changing their appearance.