Prince William County police, responding to questions about the Thanksgiving Day shooting deaths of an officer and a Dale City man, said yesterday they decided to storm the man's residence because they feared any delays could make the situation more dangerous.

Police said they entered the house, where they believed a suspect in the shooting of an Arlington County deputy might be hiding, without knowing who was inside or whether anyone was armed.

When SWAT (Special Weapons and Tactics) team officers entered, authorities said, Mark Arban, 31, was at the foot of a staircase, holding a handgun. After a tense standoff in which Arban fled to the basement, police said, Arban reemerged with an assault rifle and fatally shot Officer Philip "Mike" Pennington.

Pennington became the first officer to die from gunfire in the 20-year history of the Prince William Police Department and the third Washington area officer fatally shot in a potential barricade situation in the last two years. Pennington, 35, had been with the department 11 years.

Pennington's death has left his 302-member department in mourning and raised questions about the department's tactics. Sources at two other area police departments said they normally are reluctant to burst into homes with a gunman inside unless a life-threatening situation compels them to do so. The sources said their departments generally take as much time as needed to gain information and to give a potential gunman time to surrender peacefully.

"There is no perfect way to approach any of these situations," said Prince William Police Chief Charlie T. Deane, noting that an internal investigation is being conducted. Based on preliminary findings, "the judgment they {SWAT team officers} made I believe was sound," Deane said.

Prince William sources said police were afraid that Arban, a man described by neighbors as a recluse who lived alone, might take hostages if others were inside his single-family house. Deane also said police feared that Arban might begin sniping at officers if police did not capture him.

Prince William Commonwealth's Attorney Paul B. Ebert said his office will conduct an independent inquiry into the incident, studying legal and policy questions. He said his office routinely investigates any incidents in which deaths are involved.

Police arrived early Thursday outside Arban's house, after Arban became a suspect in the shooting and wounding of Arlington Sheriff's Deputy Daryl F. LaClair during a roadside encounter that occurred about midnight. One bullet grazed LaClair above his left eye and another struck his left elbow.

Yesterday, county police offered their first official detailed synopsis of the events that led to Pennington's death:

Officials could not say what time police arrived at Arban's house, but said it was "several hours" before a decision was made to enter the house in the 14700 block of Dodson Drive.

Some intelligence gathering was done, but when the team decided to act, "all our questions had not been answered," a county police source said. "We didn't know who was in the house . . . . We didn't know that he {the suspect} was there."

About 6:15 a.m., SWAT team members knocked on Arban's front door to execute a search warrant. They continued knocking, and after receiving no response, the group used a battering ram to force open the door.

Three officers, in full protective gear, including helmets and bulletproof vests, entered the house, and found themselves on a landing in front of a stairway leading to the basement, authorities said. After seeing Arban brandish a handgun, one officer fired a single round, police said, adding that the suspect then retreated to a basement room.

A short while later -- police yesterday were studying audiotape to determine exactly how long -- the suspect returned, this time with a high-powered assault rifle. Two officers, including Pennington, were crouched behind a shield designed to protect against gunfire from pistols and some shotguns, but not high-powered weapons. A third officer was immediately to their rear. The suspect fired a shot that pierced the shield and hit Pennington in the side of the head. The officers began to retreat while the suspect continued to advance.

Once up to the house's first floor, the suspect fired at an officer outside, who returned a single shot. Seconds later, a police sniper across the street fired a shot that is believed to have killed Arban. Police said a total of six shots were fired, three by Arban and three by officers.

Police, uncertain of the suspect's condition, then lobbed tear gas into the house. Later, after entering, they found Arban's body. Pennington was taken to Potomac Hospital, where he was pronounced dead at 7:12 a.m.

Two area experts on SWAT team tactics expressed concern that Prince William police apparently did not try to establish communications with Arban, did not retreat when given the opportunity, and that they may not have had proper intelligence about the suspect prior to making the decision to enter. Both emphasized that waiting was to the police's advantage and that there was no need to act until absolutely necessary.

One of the experts also questioned why Prince William officers, if they felt endangered when they first saw Arban with a handgun, fired only one round and allowed him to retreat to the basement.

However, other police officials said it was difficult to make conclusive judgments.

"There is no hard and fast rule on this," said Sgt. Peter Sweeney, a New York City Police Department spokesman. "Somebody at the scene has to make a judgment call."

"We would either go in like they did or we would send the robot in," Sweeney said, referring to an armed robot that the New York department has available. "Before you wait someone out, you have to ascertain someone's in there."

Chief Deane said yesterday that Arban had a history of mental problems, but declined to give specifics.

Arban's neighbors and those living near his parents in Rockville described him as a troubled man.

Catherine Chaconas, who resides on the Rockville cul-de-sac near Arban's parents' house, said when the Arbans moved in about 18 years ago, the family was wonderful, but neighbors noticed peculiarities about their son. "The boy did not come outside at all," Chaconas said. "He didn't come out to play. He didn't come out to mingle." She said the family was very concerned about the boy and tried to help over the years by sending him to special schools or to counselors.

Meanwhile, some police privately expressed concern that the media, in their quest to analyze the circumstances of the shootout, might be forgetting that the department had lost one of its finest.

"I just hope the community recognizes we lost a brave and valiant officer," said Sgt. Barry Barnard. Staff writers Patricia Davis and Gabriel Escobar contributed to this report.