The operator of the system, Colonial Pipeline, said in a statement late Friday that it had shut down its 5,500 miles of pipeline, which it says carries 45 percent of the East Coast’s fuel supplies, in an effort to contain the attack on its computer networks. Earlier Friday, there were disruptions along the pipeline, but it was unclear whether that was a direct result of the attack. Colonial’s pipeline transports 2.5 million barrels each day, taking refined gasoline, diesel fuel and jet fuel from the Gulf Coast up to New York Harbor and New York’s major airports. Most of that goes into major storage tanks, and with energy use depressed by the pandemic, the attack was unlikely to cause any immediate disruptions. In the statement, the company said that it learned on Friday that it “was the victim of a cybersecurity attack,” but it provided no details. Such an attack could involve malware that shut down its operations or ransomware demanding payment to unlock computer files or systems.