Think twice before you sign up for an online service using a fake name or e-mail address. You could be committing a federal crime.

Federal prosecutors turned to a novel interpretation of computer hacking law to indict a Missouri mother on charges connected to the suicide of a 13-year-old MySpace user.

Prosecutors alleged that by helping create a MySpace account in the name of someone who didn't exist, Lori Drew, 49, violated the News Corp.-owned site's terms of service and thus illegally accessed protected computers.

Legal experts warned Friday that such an interpretation could criminalize routine behavior on the Internet. After all, people regularly create accounts or post information under aliases for many legitimate reasons, including parody, spam avoidance and a desire to maintain their anonymity or privacy online or that of a child.

This new interpretation also gives a business contract the force of a law: Violations of a Web site's user agreement could now lead to criminal sanction, not just civil lawsuits or ejection from a site.

"I think the danger of applying a statute in this way is that it could have unintended consequences," said John Palfrey, a Harvard law professor who leads a MySpace-convened task force on Internet safety. "An application of a general statute like this might result in chilling a great deal of online speech and other freedom."

Drew, of O'Fallon, Mo., was indicted Thursday on charges of perpetrating a hoax on the popular online hangout MySpace. Prosecutors say Drew helped create a fake MySpace account to convince Megan Meier she was chatting with a nonexistent 16-year-old boy named Josh Evans. Megan hanged herself at home in October 2006, allegedly after receiving a dozen or more cruel messages, including one stating the world would be better off without her.

Drew, who has denied creating the account or sending messages to Megan, was indicted by a federal grand jury in Los Angeles on one count of conspiracy and three counts of accessing protected computers without authorization to get information used to inflict emotional distress on the girl.

Prosecutors argue that to access MySpace's servers, Drew first had to sign up for the service, which meant providing her name and date of birth and agreeing to abide by the site's terms of service. Those terms bar false registration information, solicitation of personal information from anyone under 18 and use of any information gathered from the Web site to "harass, abuse, or harm another person."

By using a fictitious name, among other things, Drew violated MySpace's terms and thus had no authority to access the MySpace service, prosecutors charged.

"Clearly the facts surrounding this matter are awful and very upsetting, and I certainly understand the instinct of wanting justice to be served," Palfrey said. "On the other hand, this complaint is certainly unusual."

Drew's lawyer, Dean Steward, said Thursday a legal challenge to the charges is planned. Missouri authorities said they investigated Megan's death but filed no charges because no state laws appeared to apply to the case.

Andrew DeVore, a former federal prosecutor who co-founded a regional computer crime unit in New York, said Friday the interpretation raises constitutional issues related to speech and due process _ in the latter case, because it doesn't allow for adequate notice of when using an alias online is criminal.

Because corporations would end up setting criminal standards, a completely legal act at one site could be illegal at another, said DeVore, who has no direct involvement in the case.

"What clearly is going on is they couldn't find a way to charge it under traditional criminal law statutes," DeVore said. "The conduct that she engaged in they correctly concluded wouldn't satisfy the statute. Clearly they were looking for some other way to bring a charge."