Multiple bomb threats against U.S. aircraft were made on Tuesday, according to NBC News. Government sources said the threats were not credible.

The threats, five in total, were similar to chemical weapon threats against aircraft last week. It was determined that the claims were fake, and there was speculation that the threats could have been made by an ISIS "lone wolf," NBC News reported.

A U.S. Airways flight and its passengers were searched after the plane landed Tuesday morning at Philadelphia International Airport.

The airport confirmed there was an ongoing police investigation after Flight 648 from San Diego landed as scheduled, with 93 people aboard.

"The TSA Operations Center in Washington, DC had received a phone threat stating that there was an explosive device on the plane," Philadelphia Police Chief Inspector Joe Sullivan told NBC News. "Out of an abundance of caution" the airport declared a bomb threat and moved the plane to a remote area.

In addition to the U.S. Airways flight, threats were made on a Delta flight to Atlanta, a United flight to Chicago O'Hare, a Korean Air flight to San Francisco and a Volaris Air flight to Gudalajara, Mexico, according to government sources.

All planes except the Korean Air flight are safely on the ground—that one is set to land around 2:22 p.m. ET.

Read More FAA briefly halts all United Airlines flights

The threats come after Homeland Security Secretary Jeh Johnson reassigned the acting administrator for the Transportation Security Administration after earlier ordering improved security at U.S. airports.

It followed media reports that checkpoint screeners failed to detect mock explosives and weapons in 95 percent of tests carried out by undercover agents.

Read MoreTSA chief out after agents fail airport breach tests