Three students, who worked in summer jobs at the US space agency (Nasa), and one other person have been charged with stealing valuable Moon rocks and attempting to sell them on the internet.

[We] must be cautious that this deal is handled with delicacy in that [we are] not publicly exposed

E-mail sent to FBI investigators

Undercover agents from the Federal Bureau of Investigation and the Nasa Office of the Inspector General arrested the four, aged between 19 and 26, after recovering a safe containing samples worth more than one million dollars.

The safe had been stolen from the Johnson Space Center in Houston, Texas.

The FBI were alerted to the theft following an e-mail tip-off, and since May this year had communicated with a group claiming to offer the "priceless Moon rocks" - which were collected by astronauts on the Apollo missions during the late 1960s and early 1970s - for sale.

Three of the suspects have been charged with conspiracy to commit theft of government property and transportation of stolen property, while the fourth is charged with conspiracy.

The three students who worked at Nasa have also been fired.

Sting operation

An advert had been placed on the Mineralogy Club of Antwerp's website, offering the samples for between $1,000 and $5,000 a gram, the Associated Press news agency reported.

Correspondents say Nasa is to beef up security following the theft

In the start of an elaborate sting operation, the FBI began e-mail correspondence with the accused, who freely acknowledged that the sale of Moon rocks was illegal under US law.

"As you well know, it is illegal to sell Apollo lunar rocks in the United States," one e-mail said.

"[We] must be cautious that this deal is handled with delicacy in that [we are] not publicly exposed."

Historical records of the samples were exchanged with the investigators to prove their authenticity.

Security measures

A meeting between three of the accused and FBI agents was then arranged last Saturday in a Florida restaurant, during which police say the suspects described how they had stolen the safe containing the rocks and loaded it into a sports utility vehicle.

They were then arrested, with the fourth alleged conspirator taken into custody in Houston the same day.

Two have since been released on bail while two others are being held in prison awaiting court hearings.

Nasa officials said they were confident that all specimens had been recovered safely, but BBC correspondents say that the space agency is planning to tighten security following the theft.