The discovery on New Year's Day of a woman's body on the grounds of the estate used by Queen Elizabeth II and England's royal family is now being treated as a murder investigation, British police said today.

The expanded investigation comes one day after a dog-walker made the discovery, finding human remains in a woodland area at Anmer, a tiny village northeast of London that is also part of the Sandringham estate.

"We are at the very early stages of the investigation and it could be a complex inquiry," Detective Chief Inspector Jes Fry said at a news conference today at the queen's Sandringham Estate in Norfolk, England.  "The circumstances suggest this is a murder case and we are looking at missing persons reports and cold cases both locally and nationwide."

The human remains were found near the Royal Stud where the queen oversees the breeding and training of race horses, and less than three miles from the queen's main residence, Sandringham House.

"The body had been there for some time," Fry said, adding that authorities were doing a post-mortem today on the body.

Sandringham House has served as a private residence for British monarchs since 1862 and is a favorite of the royal family's as a holiday retreat.  The royals, including Prince Charles, Prince William and Kate Middleton, had g athered there with the queen and Prince Philip to celebrate Christmas this year.

The queen and Prince Philip had joined other royals in attending the traditional New Year's Day service at the Church of St. Mary Magdalene on the estate hours before the discovery was made.

The royals have not spoken publicly on the discovery, but police reportedly alerted them to the discovery Monday night. "My understanding is that the queen is being kept informed of these developments when there is news to tell her," Duncan Larcombe, royal editor for the UK's Sun newspaper, told ABC News.

In addition to the queen's main residence, Sandringham Estate is spread across nearly 20,000 acres and includes sitting tenants, cottages, villages, horse grounds and a fruit farm, all managed by a staff of more than 100 people.

The grounds are particularly busy this time of year with extra security personnel on hand to guard the royal family from the flock of tourists who travel to the estate in hopes of catching a glimpse of the family.

"It's possible that somebody involved with the queen's staff may know something, may have seen something," Larcombe said.  "It seems quite extraordinary that it wasn't until a dog-walker discovered this body that something was found."

The location of the body discovery, Anmer, is  home to several dozen people, and is completely accessible to the public at all times.  It is also said to be a favorite spot of Prince Philip's.

"The area where the body was found is next to a small village on the estate, which is one of Prince Philip's favorite hunting grounds, where he likes to go out this time of year," Larcombe said.

While the murder investigation might be the first to happen directly on the grounds of a royal palace, the storyline is eerily similar for the queen and her family.

The body of Robert James Moore, a U.S. man described as a loner and obsessed with the queen, was discovered in March on an island in St. James's Park near Buckingham Palace, roughly three years after Moore was believed to have died.