The Internal Revenue Service still doesn’t know what caused some of its systems to shut down Wednesday, but the problem didn’t appear to involve hackers, said a person familiar with the matter, who asked not to be identified because the matter remains under investigation.

The shutdown appears to have involved a hardware failure and nothing else, the person said. There is no indication that any external factors had any role in the incident, he added.

The agency said in a statement that while several tools and applications went back online Thursday morning, including the “Where’s My Refund” feature on its website, it still can’t accept electronic filings of individual and business returns. The agency said it was still examining the underlying cause of the outage.

On Wednesday, Matt Leas, an IRS spokesman, said a “power or electrical issue” caused the failures. He provided no additional details. Officials say they don’t anticipate major disruptions to the payment of income-tax refunds and expect that 90 percent of taxpayers will get refunds within 21 days.

IRS officials forecast that more than 80 percent of income tax returns will be filed electronically this year. The deadline for most individuals to file their federal income taxes for the 2015 tax year is April 18.

Taxpayers who want to e-file can prepare their returns as normal and send them to their e-file provider, the agency said, but the service providers will have to hold the returns until the IRS begins accepting them again. Those who have already filed returns don’t need to take any additional steps, according to an agency news release.

The system failure occurred at an IRS center in West Virginia, according to the person with knowledge off the matter. Agency officials were trying to determine if any other facilities were affected.

The IRS website lists several facilities in West Virginia; it wasn’t clear which might have been affected. IRS sites there include at least two “enterprise computing centers” in Martinsburg and Kearneysville and the Beckley Finance Center in Beckley. The finance center, part of the IRS’s chief financial office, processes payments for manual transactions and electronic payment files and helps handle the agency’s general ledger, according to the website.