Federal prosecutors investigating how the anti-secrecy Web site WikiLeaks obtained large amounts of classified data are exploring potential criminal conspiracy charges under statutes that outlaw the theft of government property and unauthorized access to a computer as well as charges under the Espionage Act, according to a legal document.

Although it is unclear whether charges will be filed, an April 21 letter signed by Neil MacBride, the U.S. attorney for the Eastern District of Virginia, appears to offer the first official glimpse of the government’s thinking in a high-profile investigation into how WikiLeaks and its founder, Julian Assange, obtained sensitive material that it has posted on its site.

The letter accompanied a subpoena delivered this week to an individual in Boston — one of a number of individuals whom investigators have pressed or tried to press for information on WikiLeaks and who have been served with subpoenas this week. A copy of the subpoena was provided to The Washington Post with the name redacted.

Though the letter does not name WikiLeaks or Assange, sources said the subpoena was issued in relation to the probe.

The letter makes clear that an array of charges are being considered, in part, experts said, to avoid First Amendment challenges that would arise with a prosecution of WikiLeaks under the Espionage Act. That 1917 law makes it a crime to “communicate or transmit” sensitive information to an unauthorized party, and using it would probably set up a battle over an individual’s right to speak freely.

“If the Justice Department concludes that a crime has been committed, it will twist itself like a pretzel to avoid using the Espionage Act, not only because it is old and vague but because it raises a number of First Amendment problems for prosecutors,” said Abbe D. Lowell, a Washington defense attorney who has handled leak cases.

U.S. officials would not comment on any subpoenas but indicated that prosecutors are likely to carefully weigh any decision to file charges under the Espionage Act, in part because of First Amendment concerns.

“The Justice Department has decided to attack on many fronts at once,” said Assange, in a phone interview from London. One reason, he alleged, “is because it is difficult to extradite someone for espionage, espionage being a classic political offense, and most extradition treaties have exemptions” for political acts.

He blasted the investigation, saying, “It is quite wrong to go after publishers and journalists for performing their work.”

Any prosecution of Assange or WikiLeaks would be separate from a possible court-martial of Pfc. Bradley Manning, a 23-year-old soldier jailed on accusations he leaked the material.

In the WikiLeaks investigation, prosecutors have sought personal Twitter account information from Assange, Manning and several others linked to WikiLeaks.

The recipients are not the targets of the probe, sources said.

The April 21 letter, first reported by Salon.com, indicated that the individual served with the subpoena was to appear next month before a grand jury to answer questions concerning “possible violations of criminal law.” Possible violations include conspiracy to “knowingly [access] a computer without authorization” and to “knowingly [steal] any record or thing of value” belonging to the government.

“What they are trying to do is find proof that the WikiLeaks people were in a conspiracy with the leaker to get the information,” Lowell said. “If WikiLeaks is involved in the theft or improper access to the information, that’s not protected under the First Amendment.”

Staff writer Dana Hedgpeth and research editor Alice Crites contributed to this report.