The woman was taken into custody at a highway rest stop, where she was found inside a pickup truck that matched the description of a vehicle seen near the spot where the boy's body was found Saturday in South Berwick, Maine. The pickup is registered to Julianne McCrery, of Irving, Texas, police said.
A telephone tip led police to the woman Wednesday at a highway rest stop in Chelmsford, Massachusetts State Police spokesman David Procopio said.
Several troopers who responded engaged the woman in conversation, and she agreed to go to state police barracks for questioning. She has not yet been charged with in the boy's death, which has been described as suspicious by Maine officials.
After several hours of questioning, Procopio said state police decided to have the woman taken to a hospital for a medical evaluation.
Maine State Police said investigators believe they know the boy's identity, but won't release information until the state medical examiner's office makes a positive identification and the boy's family is notified.
The voicemail was full for a Texas phone listing for McCrery.
State police Lt. Brian McDonough told reporters that detectives were investigating new information Wednesday but referred all questions to Deputy Attorney General William Stokes. Stokes, who coordinates homicide investigations in Maine, told The Associated Press the case was being investigated as a "suspicious death," but he wouldn't say it's being treated as a homicide.
Investigators have fielded more than 200 tips since the boy's body was found. Police also conducted DNA tests on the body, released a detailed photo of the boy's sneakers and notified Interpol.
On Wednesday, the police discovery of the woman set off a rapid-fire chain of events.
By day's end, New Hampshire investigators had taken over the case. New Hampshire officials advised that there would be no comment until Thursday, Maine officials said.
If New Hampshire decides to seek charges against the woman, she would be considered a fugitive from justice in Massachusetts and would not be allowed to go home from the hospital, Procopio said.
New Hampshire's senior assistant attorney general Jeffery Strelzin confirmed that the state is involved in the investigation, but had no further comment.
It's extremely unusual for a missing child to go unreported. Similar cases happened only twice over the past two years, said Ernie Allen, president of the National Center for Missing and Exploited Children.
"In the vast, vast majority of these, there's someone, a parent or grandparent, searching for that child," Allen said Wednesday.
In April 2010, the body of a 6-year-old boy was discovered near a lake in north Texas and his grandmother was eventually charged with murder; in May 2009, a 3-year-old boy's body was found buried at a playground in Albuquerque, N.M., and his mother was eventually charged with killing him.
In both cases, family members failed to report the boys missing.
"In cases where children are murdered or disposed of, typically the perpetrator is the one who should be reporting the child missing," Allen said.
In Maine, the case has led to an outpouring of emotion. Several hundred people attended a candlelight vigil in the boy's memory Tuesday night in front of the South Berwick town hall.
Near where the boy was found, people have placed three crosses, dozens of stuffed animals, candles, flowers, a baseball and other children's items. A framed piece of paper says, "God Bless This Little Boy."
Bruce and Laurie Ralph, who live down the street from where the body was found, placed a stuffed animal on the site.
"The whole community has come together and has feelings for this boy, who nobody seems to know who he is," Laurie Ralph said Wednesday as she and her husband visited the site. "You hear of missing children all the time, but when it happens in your hometown - and on your own street - it's scarier."
Associated Press writers Clarke Canfield in Alfred, Maine, David Sharp in Portland, Maine, and Kathy McCormack in Concord, N.H., contributed to this report.