A large water transmission line ruptured early Friday in North Arlington, closing schools, causing a traffic headache and leaving thousands without running water on both sides of the Potomac River.

Crews responded to the break at about 4 a.m. on Glebe Road near the Chain Bridge after residents reported water outages and low water pressure. Service was restored later in the morning, but more than 100,000 customers in Arlington and the District were advised to boil water as a precaution until tests show it is safe to drink.

The Arlington Department of Environmental Services issued the boil-water advisory for about half the county, suggesting that residents boil water for three minutes before drinking. Across the Potomac River, D.C. Water officials said customers in parts of upper Northwest Washington also should boil water because of a loss of pressure tied to the Virginia water main break.

Katie O’Brien, a spokeswoman for the Arlington Department of Environmental Services, said it wasn’t known what caused the 36-inch pipe to fracture.

“There’s no clear reason why the pipe broke,” O’Brien said. “Weather and age are always factors. We just don’t have a clear cause at this time. I don’t know if we’ll ever know."

More than 100,000 residential and business customers in Arlington were under a boil-water advisory that is likely to remain in place until at least Sunday as water is tested. D.C. officials said more than 6,000 customers were affected in the District, where the advisory also will stay in place until at least Sunday. D.C. Water officials worked Friday to flush the water system and will decide when to lift the advisory after two rounds of testing.

Officials noted that filtered tap water also should be boiled, as most household water filters don’t remove bacteria or viruses. The advisory was posted as a precaution, according to water officials, with no indication that water was contaminated.

The ruptured pipe, which was installed in 1965, is much larger than most in the county, where water lines range from 1 inch to 48 inches in diameter. Most water mains in Arlington are 6 or 8 inches in diameter. More than 60 percent of Arlington’s mains are at least 50 years old.

D.C. Water spokesman Vincent Morris said water restoration to the District was “contingent on work happening in Arlington” because the systems are connected, as both get water from the Washington Aqueduct.

A transmission main much larger than neighborhood water mains runs under the Potomac River near the Chain Bridge, bringing water from the District into Arlington, Morris said. When it broke, smaller distribution lines on the D.C. side emptied, causing low pressure that led to the water-boil advisory east of the Potomac.

“Our crews have been working all night, but we are hamstrung by the break there,” he wrote in an email, referring to work in Arlington. A D.C. Water map of areas with low pressure in the District included neighborhoods roughly from Chevy Chase to Tenleytown and to parts of Wesley Heights.

The outage disrupted morning routines for thousands of residents in Arlington, McLean and the District who didn’t have water, didn’t have classes or had to find alternate routes to work.

Arlington County Public Schools announced at 7 a.m. that all schools would be closed Friday, saying water pressure was affected across the county.

The Chain Bridge was closed for several hours around the morning rush but was reopened by 10 a.m., about the same time Route 123, known as Chain Bridge Road, also reopened. Arlington officials said Glebe Road — which was severely damaged as crews worked to repair the pipe — will remain closed “until further notice” between Route 123 and Military Road.

Lauren Brewer, who lives in the Court House neighborhood, said she wasn’t aware of the boil-water advisory until she had left her house Friday morning.

“I get to work and read the news, and see that I probably should have boiled my water,” she wrote in an email. “Thankfully, I read about the advisory and warned my husband since he gets up later than me so that he and our pets don’t drink unboiled water.”

Restaurants in the advisory area brought in bottled water as faucets sat idle. Arlington high school football games continued as scheduled, but concessions included only prepackaged items and bottled water.

Patricia Burke, who lives in Arlington’s Ashton Heights neighborhood, said her taps were “spitting” when she turned them on about 7:30 a.m. She boiled water for her dog, she said, and cleaned out its bowl with boiled water.

“A minor inconvenience for me since I’m retired and have time to fool with stuff,” she wrote in an email. “Can’t imagine how families with kids and jobs are handling this.”

Steve Kramer, who lives in the Cherrydale neighborhood, said he awoke around 6 a.m. to find his water “spurting from the faucet.” Pressure was restored around 7:30 a.m., he said, but he was grateful he didn’t have to commute.

A traffic app redirected his partner from the closed Chain Bridge to the clogged Beltway for his morning trip to Bethesda.

“That’s going to be a challenge,” Kramer said.