On a Tuesday after a network breach, the Colonial Pipeline’s scheduling system prevented customers from planning upcoming shipments on Americas largest fuel pipeline, two well-known market sources say.

The disturbance has been caused by the company’s efforts to harden its system as service is restored after a week-long cyber attack, Colonial said, and was not a result of the network reinfection.

Colonial did not confirm immediately if its scheduling system was back online. One of the market sources said that the network was still moving slowly.

scheduling system was back online on Tuesday after a network outage earlier in the day prevented customers from planning upcoming shipments on the biggest U.S. fuel pipeline, two market sources familiar with the matter said.

The disruption was caused by efforts by the company to harden its system as it restored service following a week-long outage due to a cyberattack, Colonial said, and was not the result of a reinfection of its network.

Colonial did not immediately confirm whether its scheduling system was back online. The network was still moving slowly, said one of the market sources.

Last week’s closure of the 5,500-mile (8,900-km) system was the most disruptive cyberattack on record, preventing millions of barrels of gasoline, diesel and jet fuel from flowing to the East Coast from the Gulf Coast.

After the ransomware attack forced Colonial to shut its entire network, thousands of gas stations across the U.S. southeast ran out of fuel. Motorists fearing prolonged shortages raced to fill up their cars.

Colonial has been using its shipper nomination system to schedule batches of fuel deliveries to bring flows back to normal.

Colonial’s shipping nomination system is operated by a third party, privately-held Transport4, or T4, which handles similar logistics for other pipeline companies. T4 could not say when the issue would be fixed, and did not comment on whether its systems for other pipelines were affected.

As of Tuesday, more than 10,400 filling stations were still without fuel, according to tracking firm GasBuddy, down from more than 16,000 at the peak last week.

GasBuddy said that gas outages fell below 50% in North Carolina, one of the most hit countries, on Tuesday. South Carolina, Virginia and Georgia were also under 50% outages.

Less than 70% of Washington, D.C. gas stations were still lacking fuel, down from approximately 90% over the weekend.

The GasBuddy’s Patrick De Haan told me on Tuesday “The number of stations without gasoline is expected to decline by less than 10,000 today.