Authorities are trying to determine how the whereabouts of two Colorado boys went unnoticed for up to a decade.  

Law enforcement sources told FoxNews.com Thursday that authorities are now searching for Austin Eugene Bryant and Edward Dylan Bryant, who haven't been seen for at least 10 years.

Colorado Springs sheriff's spokeswoman Lari Sevene said in an interview that the boys -- ages 7 and 9 at the time of their disappearance -- were living with their adoptive parents in the Monument area north of Colorado Springs. 

The boys were enrolled in a school but had been taken out to be home-schooled, Sevene said.  

Edward Bryant, 58, and Linda Bryant, 54, the boys' adoptive parents, were arrested in Texas and extradited to Colorado. Both are being held in the El Paso County jail on $1 million bail each, facing theft, conspiracy, forgery and other charges stemming from collecting subsidies from the El Paso County Department of Human Services. The two have so far not been charged in the disappearances of Austin and Edward.

Sevene said the investigation began when the The El Paso County Sheriff's Office in Colorado received a "suspicious incident report" on Jan. 22  involving the disappearance of Austin Bryant.  

Upon further investigation, authorities found that Austin was last seen some time between 2003 and 2005. Investigators then discovered that a second child, identified as Edward Dylan Bryant, was also unaccounted for. Edward is believed to be missing since as early as 2001. 

No missing persons report for the boys was ever filed, leading investigators to wonder how the children's whereabouts went unnoticed for so long. 

"That’s part of what we’re trying to determine -- to see why that wasn't reported," Sevene said. "There are a lot of questions that need to be answered."  

The Bryants lived in the Monument area north of Colorado Springs between 1999 and 2005. Edward Bryant was living in Denton, Texas, at the time of his arrest, while Linda Bryant was living in Gainesville, Texas. 

"I don't really remember them, but they seemed like shady people," one of the Bryants' neighbors told kktv.com. "Something just wasn't quite right."

The sheriff's office plans to conduct a neighborhood canvass and follow up on other leads to find the children, who would now be 15 and 18. 

Anyone with information on the boys is being asked to call the El Paso County Sheriff's Office at 719-390-5555.

The Associated Press contributed to this report