A massive water main break in Prince George's County shut down the inner loop of the Capital Beltway for hours Monday morning, forced the closure of a federal agency and required Maryland's suburban water utility to issue a boil-water order for 400,000 customers.

The order affects Washington Suburban Sanitary Commission customers in an area roughly from the District line to Route 301 and from just south of Central Avenue to Indian Head Highway.

Officials said the order was a precaution because of the loss of pressure in the system, indicating an increased possibility of contamination. 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.

Lyn Riggins, a WSSC spokeswoman, said it was uncertain how long the boiling advisory will be in effect. "We know it's an inconvenience, and we'll lift it as soon as we can," she said.

Officials said the order will last until pressure is restored in the system and water quality has been confirmed through tests.

WSSC General Manager Jerry N. Johnson said during a news conference at the site that it was unclear how long repairs will take. But the problem "will certainly cost over $1 million" to fix, he said.

An investigation will be necessary to determine the cause of the break, Johnson said.

The U.S. Census Bureau closed its headquarters in Suitland on Monday because the building had no water, and Bishop McNamara High School in Forestville also closed.

The break caused damage to a nearby one-story office building. Several vehicles in a parking lot were damaged; at least two cars were overturned and a pickup truck smashed in the torrent.

Riggins said the pipe that ruptured was a major distribution main, 54 inches in diameter.

"It still looks like white-water rapids behind there, but it's greatly reduced from this morning," Riggins said late Monday morning, referring to the industrial area adjacent to the interstate where the water main broke. The area is a few blocks south of Central Avenue.

WSSC officials said the break occurred about 3:50 a.m. For about six hours, cars were rerouted from the Beltway to local streets, but by Monday afternoon most of the lanes were flowing again. Road crews treated the highway to prevent the formation of ice.

Members of Maryland's congressional delegation said the massive break highlighted widespread problems in the area's infrastructure.

Johnson said the 40-year-old pipe was last inspected in 1999. He toured the site of the break with Rep. Donna F. Edwards (D-Md.).

Residents and local governments need the federal government to "be a partner" to adequately address the infrastructure issue, Edwards said.

"It stops our competitiveness; it shuts us down; commuters can't get to work," she said.

Sen. Benjamin L. Cardin (D-Md.) renewed calls for the reauthorization of the Water Infrastructure Financing Act.

"The major water main breaks that have become near epidemic in our region and elsewhere tell us that major parts of the system are too old and too frail to hold together much longer," he said in a statement.

This is not the first time a water main break has caused major problems for the WSSC. Officials shut down a 96-inch main last summer and imposed water restrictions after an inspection indicated there could be weaknesses in the pipe. In late 2008, a 66-inch main burst along River Road in Bethesda, requiring motorists to be rescued by helicopter and firefighters in boats.

Johnson said he could not speculate about whether frigid temperatures caused the break.

WSSC, which serves 1.8 million customers in Prince George's and Montgomery counties, experienced 674 breaks last month - an all-time record, officials said.

Beltway drivers weren't the only ones who had a difficult morning commute.

Switch and signal problems delayed riders on MARC trains. Officials were unsure whether the failures were related to the cold.

MARC riders were given the option of riding Metrorail for free - but the subway had its own problems. Cracked rails on the Blue and Red lines caused delays for much of the morning.

Metro spokeswoman Lisa Farbstein said the problems "were partially related to the [cold] weather and partially related to the fact that in previous years we have not done more to maintain the rail system in a state of good repair."

Staff writers Ann Scott Tyson and Ed O'Keefe contributed to this report.