Residents and businesses in a large portion of Prince George's County should continue to boil drinking water until at least Thursday, the earliest that final test results to determine the water's safety will be available, a Washington Suburban Sanitary Commission official said.

WSSC officials put the boil-water advisory into effect Monday after a major water main blew apart near the Capital Beltway, destroying a church and two businesses at a Capitol Heights office park and leaving 400,000 people south of Central Avenue (Route 214) and Landover Road (Route 202) without water for much of the day. Officials said all water has been restored. The advisory for that area is a precaution because the break caused the system to lose pressure, leaving it more vulnerable to contamination.

Officials sent water samples to the lab Tuesday afternoon, WSSC spokesman John C. White said. Because it takes 18 hours to get test results, the first results will be available midday Wednesday. The advisory could be lifted Thursday if those results, plus a second round from tests to be done Wednesday, come back clean, he said.

While restaurants served bottled water and residents turned up the stove before brushing their teeth, some tenants at the Central Hampton Business Park picked through what little the water left behind. Windows were smashed, doors torn off their hinges and walls ripped out.

"It looks like a hurricane hit," said Marian Blair, the business park's property manager. "I can't believe water can do that much damage."

She said overnight workers at a postal facility next door told her they hit the floor when the pipe exploded about 3:50 a.m. Monday because it sounded like a bomb blast.

Stephanie Stratford, pastor of the Ekklesia Family Life and Worship Center, said she still hasn't found bookcases and books the water carried away. The force of the flood pushed a sofa and conference table through a doorway and part of a wall and into the sanctuary 30 feet away, she said.

"There is nothing salvageable," Stratford said. "I'm just so excited and feeling blessed that no one was there when the pipe broke. I'm certain we would have been killed."

White said officials are still trying to determine what caused the 54-inch water main to burst. Crews inspected the pipe Tuesday and will replace at least two sections Wednesday, he said. The repairs will not affect users because water has been diverted to another pipe, White said.

The 40-year-old pipe that broke was inspected in 1999 and wasn't scheduled for another inspection until 2013, White said. It did not have the fiber-optic equipment that the WSSC now installs during inspections of large pipes to detect sounds indicating serious decay and warning of an impending break.

Like other utilities nationwide, the WSSC has come under scrutiny because decaying underground pipes are bursting with growing frequency.

Andrews Air Force Base resumed full operations Tuesday after telling all but essential employees to stay home Monday because the base had very low or no water pressure, said spokesman Eric Sharman.

"If our restrooms aren't working, we can't have thousands of people working here," Sharman said.

On Tuesday, five large water trucks were parked outside certain facilities, he said.

At the Fish Market restaurant in Clinton, Sherry Giovannoni increased her regular ice delivery order and served bottled water. She said she had extra hand sanitizer available for customers who weren't sure whether washing their hands in the restroom was safe.

"You don't want to take a chance on anyone getting sick," Giovannoni said.

Water for drinking, ice, washing dishes, brushing teeth and food preparation should be brought to a rolling boil for one minute and cooled, WSSC officials said. The order also applies to water used by pets.