The cyberattack that forced the closure of the top U.S. fuel pipeline threatens to disrupt gasoline supplies for millions, as the conduit’s owner estimated Monday that restoring service would take at least through week’s end. The new timeline for the Colonial Pipeline, a 5,500-mile system from Texas to New Jersey, came as the Federal Bureau of Investigation said it believed the attack involved a criminal gang with Eastern European ties known as DarkSide.

Colonial Pipeline Co. said Monday it hoped to substantially restore service on the pipeline, which it shut Friday, by the end of this week. The Colonial Pipeline transports about 45% of the fuel consumed on the East Coast, according to the company’s website. The Alpharetta, Ga.-based company said that restoring its network, in coordination with the Energy Department and other federal agencies, will take time, and that it was bringing segments back online in stages.

DarkSide created the malicious code that resulted in the Colonial Pipeline’s shutdown, FBI officials said Monday. The organization is a relatively new hacking group that Western security researchers say is likely based in Eastern Europe, possibly in Russia.

The group posted a statement earlier Monday claiming that its sole goal was to make money and denied it was connected to a foreign government. It didn’t mention the Colonial Pipeline or how much money was being demanded. “We are apolitical, we do not participate in geopolitics,” the group said in the statement.

The Biden administration issued an emergency waiver extending hours for truck drivers delivering fuel across 17 states, including several across the southeastern U.S. that depend on the pipeline for much of the fuel they consume.

President Biden said Monday that he is prepared to take additional action, depending on how much time is needed for the pipeline to resume operations. He said ransomware was a growing problem in need of a global response and that more investment in critical infrastructure was necessary to safeguard critical systems from debilitating cyberattacks.

Mr. Biden and others said the Russian government didn’t appear to have a hand in the attack, but he criticized Moscow for tolerating criminal hackers within its borders.