Even the Buddha of compassion might have been distressed to be on the receiving end of the diatribes that William Lawrence Cassidy is accused of posting on Twitter.

They certainly rattled Alyce Zeoli, a Buddhist leader based in Maryland. Using an ever-changing series of pseudonyms, the authorities say, Cassidy published thousands of Twitter posts about 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. PS Have a nice day."

Those relentless tweets landed 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 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.

Federal authorities say Cassidy's Twitter messages caused Zeoli "substantial emotional distress" and made her fear for her life, so much so that she once did not leave home for 18 months and hired guards to protect her residence.

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

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," Volokh argued. Certainly 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 defence 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 Cassidy lived across the country in California and is not accused of getting anywhere close to Zeoli. He is now in jail in Maryland pending trial. In support of a defence motion to dismiss the case, the electronic frontier foundation, an advocacy group in San Francisco, appealed 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 Brooklyn, 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 Zeoli said she declined to be interviewed for this article. According to the FBI and Zeoli's lawyer, Cassidy also claimed to be a reincarnated Buddhist when he joined Zeoli's organization, Kunzang Palyul Choling, in 2007. He signed up using a false name and claimed to have had lung cancer, they said. 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. Cassidy left. Then came the relentless tweets. "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.