An international air traveler is cleared by a U.S. Customs and Border Protection Officer inside the U.S. Customs and Immigration area at Dulles International Airport.

U.S. Customs and Border Protection said Friday evening that systems around the country were being restored after an earlier outage prevented U.S. immigration officials from processing thousands of arriving passengers during a busy summer travel day.

@CBP: The affected systems are coming back online and travelers are being processed. CBP will continue to monitor the incident. There is no indication the disruption was malicious in nature at this time.

It wasn't immediately clear what caused the problem at U.S. Customs and Border Protection, but the agency said it will continue investigating. There was no indication that the system was intentionally derailed, according to a CPB official.

CBP officers used "alternative procedures" to process travelers arriving from abroad, the agency said.

Some travelers posted images on social media of long lines at checkpoints at airports in Washington, D.C., and Houston.

@RebekahKTromble: Nationwide outage of US CBP computer systems. Easily 5,000+ passengers in line at Dulles.

One traveler tweeted that passengers on her flight from London weren't allowed to deplane upon arrival in Boston because of the issue.

@savvybostonian: Landed at @BostonLogan from London 20+ mins ago & we cannot leave the plane due to an issue with the computer systems at immigration. We're hearing it is a nationwide issue and not unique to Logan Airport so they have to process everyone manually. Anyone have any news on this?﻿