Nassau's top cop is set for a legal battle against an anonymous Facebook user after the latter has been critical of the Nassau County Police Department online.

Acting Commissioner Thomas Krumpter issued a subpoena last month to access account information from the person behind the pseudonymous Facebook account of "Sue Reilly." The user's lawyer has filed a motion against the appeal.

"We believe he is abusing his subpoena power in an attempt to figure out who was saying bad things about him," says Vincent Grande III, the user's attorney.

Grande says his client wants to keep his identity secret and is fighting the subpoena. The lawyer filed the motion to quash the subpoena on behalf of "John Doe," arguing in part that it violates his right to free speech.

"He's entitled to have his opinion," Grande says. "And he's entitled to do it anonymously. We still live in America."

NCPD says it cannot comment on the issue because the subpoena is part of an active internal investigation, although Grande says his client is not an active police officer.

Grande says his client has deactivated the Facebook account over fears of retaliation from police.

A state Supreme Court judge will hear the case Wednesday in Mineola.