Pippa MIddleton

Detectives investigating the hacking arrested a 35-year-old man in Northamptonshire in central England, police said.

The man was arrested on suspicion of a Computer Misuse Act offense, and he is now in custody at the south London police station, the Met Police said.

Images of Kate Middleton and her children, Prince George and Princess Charlotte, were reportedly among some 3,000 images stolen when the account was hacked, according to Britain's Press Association news agency.

Both Britain's Sun and Daily Mail newspapers reported that an anonymous seller had been in contact offering the images for sale.

The Sun said the cyber thief -- who got in touch via an encrypted messaging service and used the pseudonym "Crafty Cockney" -- claimed to have images that also included Middleton at a wedding dress fitting and nude photos of her fiance, financier James Matthews.

The seller demanded "a minimum of £50,000" (around $65,000) within 48 hours, as of Friday afternoon UK time, both newspapers said.

Middleton, the Duchess of Cambridge's 32-year-old sister, announced Tuesday she is engaged to marry her boyfriend, James Matthews, a hedge fund manager.

"Miss Pippa Middleton and Mr. James Matthews are delighted to announce they became engaged on Sunday 17th July and plan to marry next year," the couple said in a statement released by the Press Association.