In April 2007, William Hallowell, an assistant in the library at the Riverdale Country School in the Bronx, sent e-mail messages to his boss. He apologized for not having been at work because of some family problems, and he wrote that he was resigning, because he also needed more time to study.

His boss, the library director, was sympathetic in her messages responding to him.

But what began as an innocent exchange of e-mail messages, Mr. Hallowell said, quickly spiraled into an Internet nightmare, with the librarian mistakenly sending another message meant for him to someone with a similar name, the recipient replying with a crude, abusive response, and the blame falling on Mr. Hallowell.

He was arrested on a harassment charge, interrogated, held in custody for more than 30 hours, and became the subject of local news articles, causing enormous embarrassment, he said in a lawsuit filed on Wednesday in Federal District Court in Manhattan.

In the suit, which names New York City and several police officers as defendants, Mr. Hallowell, 24, says that the officers ''deliberately and maliciously ignored a mountain of evidence'' that proved that he did not send the offending message.

In an interview, he added that the officers did not even seem to understand how e-mail addresses work.

''I said it to them literally, 'I don't know,' countless times. 'That's not my name.' 'That's not my e-mail.' 'I did not send that.' ''

The harassment charge was dropped last August, the lawsuit said, because of a lack of evidence, but only after Mr. Hallowell made three court appearances over four months.

Asked about the suit, a spokeswoman for the city's Law Department said only, ''We are awaiting the formal legal papers and will review them thoroughly.'' She said that the Police Department would have no separate comment.

A spokeswoman for the Riverdale Country School, which is not a defendant in the suit, also declined to comment.

But one of Mr. Hallowell's lawyers, Ilann M. Maazel, said the case showed how easy it was for innocent e-mail users to be victimized.

''This could happen to anybody,'' he said, ''if the police are going to have absolutely no competence when it comes to understanding e-mail or the Internet.''

Mr. Hallowell, who lives in Dobbs Ferry, N.Y. and works with a nonprofit Web site that focuses on politics, said that he did not even know about the lurid e-mail message until the officers who went to his apartment showed it to him.

Most of its contents cannot be reprinted in the newspaper.

One line says, ''I want your sweet body against my skin!'' There is a racist reference, talk of using prostitutes and a statement that the sender had bought a gun and considered suicide.

In his earlier exchange of e-mail messages with the librarian, Mr. Hallowell said, she used his correct e-mail address.

But the suit says that the abusive response came after the librarian inexplicably sent a message to another e-mail address that was similar but did not belong to him.

Even if she believed that address was his, the suit suggests, it was clear that the response was not from him. It came, Mr. Hallowell said, from an e-mail account linked to a Ben Hallowell.'

William Hallowell said that he vigorously denied sending the lurid e-mail message, and that he invited the officers to review the e-mail messages in his computer. He said that he also showed them the exchange of messages he had earlier had with the librarian.

It was clear, he said, that his account did not contain any with the address linked to the abusive sender.

Mr. Hallowell's suit said that after he was taken into custody, one officer ''repeatedly told him he would be found guilty and asked if he had psychological problems.''

The suit noted that local newspapers reported on the abusive e-mail message, and that after a parent at another school heard about the incident, the school was locked down briefly out of concern for students' safety.

Mr. Hallowell's lawyer, Mr. Maazel, said that to his knowledge, no one had figured out who the anonymous sender is.

As for Mr. Hallowell, who is to be married this month, he still feels shaken when he recalls the experience. ''I've never had even a speeding ticket,'' he said.

''I was totally in shock. I was waiting for the hidden cameras to come out. It was just unbelievable.''

Copyright 2008 The New York Times Company