<< Alyce Zeoli, a Buddhist leader now based in Maryland, in 1995. Mannie Garcia

They certainly rattled Alyce Zeoli, a Buddhist leader based in Maryland. Using an ever-changing series of pseudonyms, the authorities say, Mr. Cassidy published thousands of Twitter posts about Ms. Zeoli. Some were weird horror-movie descriptions of what would befall her; others were more along these lines: “Do the world a favor and go kill yourself. P.S. Have a nice day.”

Those relentless tweets landed Mr. Cassidy in jail on charges of online stalking and placed him at the center of an unusual federal case that asks the question: Is posting a public message on Twitter akin to speaking from an old-fashioned soapbox, or can it also be regarded as a means of direct personal communication, like a letter or phone call?

Twitter posts have fueled defamation suits in civil courts worldwide. But this is a criminal case, invoking a somewhat rarely used law on cyberstalking. And it straddles a new, thin line between online communications that can be upsetting — even frightening — and constitutional safeguards on freedom of expression.

In a complaint filed in federal court in Maryland, the Federal Bureau of Investigation concluded that Mr. Cassidy had published 8,000 Twitter posts, almost all of them about Ms. Zeoli and her Buddhist group, along with similar posts on several blogs.

Mr. Cassidy’s lawyers with the federal public defender’s office argue that even offensive, emotionally distressing speech is protected by the First Amendment when it is conveyed on a public platform like Twitter. Legal scholars say the case is significant because it grapples with what can be said about a person, particularly a public person like a religious leader, versus what can be said to a person.

Eugene Volokh, a law professor at the University of California, Los Angeles, offered an analogy: the difference between harassing telephone calls and ranting from a street-corner pulpit. “When the government restricts speech to one person, the speaker remains free to speak to the public at large,” Mr. Volokh argued.

Certainly Mr. Cassidy’s previous trespasses have not helped him. He has a record of assault, arson and domestic violence. According to the federal complaint, he was also convicted of carrying an unspecified “dangerous weapon” onto a plane in 1993.

But the defense has taken pains to point out that across the Internet, people post things that may cause emotional distress to others: an unkind review of a book on Amazon, even an unvarnished assessment by a college student on RateMyProfessors.com. They point out, moreover, that Mr. Cassidy lived across the country in California and is not accused of getting anywhere close to Ms. Zeoli. He is now in jail in Maryland pending trial.

In support of a defense motion to dismiss the case, the Electronic Frontier Foundation, an advocacy group based in San Francisco, appealed to the court to protect online expression.

“While not all speech is protected by the First Amendment, the idea that the courts must police every inflammatory word spoken online not only chills freedom of speech but is unsupported by decades of First Amendment jurisprudence,” it wrote.

Born in Canarsie, Brooklyn, Ms. Zeoli is considered to be a reincarnated master in the Tibetan Buddhist religious tradition, and is known to her followers as Jetsunma Ahkon Lhamo. She is an avid Twitter user, with 23,000 followers. A representative for Ms. Zeoli said she declined to be interviewed for this article.

According to the F.B.I. and Ms. Zeoli’s lawyer, Mr. Cassidy also claimed to be a reincarnated Buddhist when he joined Ms. Zeoli’s organization, Kunzang Palyul Choling, in 2007. He signed up using a false name and claimed to have had lung cancer, they said. Ms. Zeoli’s organization cared for him and, briefly, even appointed him to its executive team. The relationship soured after they came to doubt his reincarnation credentials and found that his claims of cancer were false. Mr. Cassidy left. Then came the relentless tweets, they said.

“A thousand voices call out to (Victim 1) and she cannot shut off the silent scream,” read one in the summer of 2010, as redacted in the criminal complaint.

“Ya like haiku? Here’s one for ya. Long limb, sharp saw, hard drop,” read another.

Shanlon Wu, a former federal prosecutor who is representing Ms. Zeoli, likened the tweets to “handwritten notes.” Every time Ms. Zeoli blocked the messages, more appeared from a different Twitter account. Ms. Zeoli for some time stopped using Twitter altogether.

“She felt constantly attacked and monitored by these anonymous people, and the attacks went on whether or not she was online,” Mr. Wu said by e-mail.

Twitter, in response to a subpoena, revealed the Internet protocol address of the computer used to post the messages. The authorities found Mr. Cassidy at home in a small Southern California town called Lucerne Valley. Similar rants were posted on blogs that law enforcement authorities say they traced to him. Twitter did not respond to a request for comment.

The case is an example of the many ways in which the law is having to wrestle with behavior on new, rapidly changing modes of communication.

Similar issues have come up in state courts: a boy who hacked into the Facebook account of an acquaintance was charged with felony identity theft, and a student who bombarded a professor with mean e-mail was accused of disturbing the peace.

“Technology creates new ways for people to interact with each other,” said Eric Goldman, a law professor at Santa Clara University in California. “You have to figure out if old law maps to new interactions.”

Twitter is an especially vexing new tool. It prompts ordinary people who use it to create public personas and it can put celebrities, including religious leaders, in direct contact with a large and sometimes unruly following, including some who insist on using pseudonyms.

“How do you cope with them?” Mr. Goldman wondered aloud. “Do you just block them? Or do you make a federal case out of it?”