An intruder who initially identified himself as an Iranian national was arrested Tuesday for trespassing after guards stopped him within a highly secure area at one of the Navy's most sensitive electronic warfare centers in southern Maryland, according to law enforcement and Navy officials in the area.

The intruder, who told law enforcement officials that he was from Tehran, was discovered inside the grounds of the Naval Electronic Systems Engineering Activity near St. Inigoes, Md., at about 6 p.m. Tuesday, according to southern Maryland law enforcement officials and sources at the naval warfare facility.

Law enforcement officials are trying to determine the man's motive. They said he was not armed.

Meanwhile, authorities at the nearby Naval Air Station at Patuxent River said security has been tightened at naval facilities throughout Maryland, Virginia and the District of Columbia.

The Navy conducts some of its most sensitive electronics and weapons testing at the southern Maryland facility.

Officials said the man was turned over to the St. Mary's County sheriff's department after interrogation by agents of the Naval Investigative Service.

A sheriff's department official confirmed last night that the man, who appeared to be in his thirties, was detained without incident by the Naval Air Station police and was held Tuesday night by the sheriff's department.

The man was taken to Hyattsville yesterday to appear before a federal magistrate on charges of trespassing and vagrancy.

Law enforcement officials said they do not know how the man penetrated what they described as the highly secure boundaries of the testing center.

Law enforcement officials said the man had been "cooperative," but said he changed his name and identity several times after first saying he was from Tehran.

Naval sources at the facility said the arrest has raised new concerns over possible attacks from Iran within the United States as a reaction to U.S. Navy escorting operations for reflagged Kuwaiti

oil tankers in the Persian Gulf.

Kuwait is an ally of Iran's enemy Iraq.