The Internal Revenue Service’s electronic system that allows Americans to submit their tax returns online has partly failed Tuesday, complicating filing for the millions of taxpayers attempting to meet the government’s midnight deadline.

“On my way over here this morning, I was told a number of systems are down at the moment,” IRS Acting Commissioner David J. Kautter told lawmakers at an oversight hearing Tuesday. “We are working to resolve the issue and taxpayers should continue to file as they normally would.”

Kautter said the problem was “probably” the result of internal technical problems. But he said he did not know the exact cause and could not rule out external interference — including from a cyberattack.

The full extent of the failure could not immediately be determined. Kautter said the agency was struggling to accept returns from the widely used tax software program TurboTax and the massive tax preparation company H&R Block.

Kautter said taxpayers could continue to use those systems to file, but he said some returns right now were not going through to the IRS.

The IRS web site reported at midday Tuesday that the agency’s Modernized eFile System was down. “We are working [on] this as a priority,” the site said.

Kautter said he did not yet know how many people would be affected, but he said “it could be millions, potentially.” The agency estimates approximately 30 million people still need to file their returns, though many of those individuals will seek extensions.

Last year, 5 million people filed their returns on the final day, IRS spokesman Dean Patterson said.

The IRS discovered the problem early Tuesday morning, Kautter said, and it plans to attempt a “hard reboot” of its system in an effort to address the failures.

Kautter said he hoped to have the problem resolved soon but that the agency would not punish people if their returns arrive late because of the glitches.

“If we can’t solve it today we’ll figure out a solution,” Kautter said. “Taxpayers would not be penalized because of a technical problem the IRS is having.”

Lawmakers also pressed the agency to accomodate people affected by the glitches.

“Tax Day is already a stressful time for millions of Americans, even when everything goes right. Given this news, I hope that the IRS will make accommodations so that every taxpayer attempting to file today has a fair shot to do so without penalty,” Rep. Richard Neal, the top Democrat on the House Ways and Means Committee, said Tuesday in a statement.

Lawmakers expressed frustration with the technical difficulties at the oversight hearing Tuesday.

“Unfortunately, it’s another example where they’re not capable of dealing with the volume,” said Sen Rob Portman (R-Ohio), who has called for reforms at the IRS partly because of the agency’s technological shortcomings. 

“This is game-day for the IRS, and it seems the IRS can’t get out of the locker room,” said Rep. Greg Gianforte (R-Mont.).

Damian Paletta and Erica Werner contributed to this report.