More than half a million aliens overstayed their temporary visas in the United States in 2015, with more than 482,000 of those individuals believed to be still residing illegally in the United States, according to a new report by the Department of Homeland Security.

Around 527,127 aliens temporally granted U.S. business and tourist visas were found to have stayed in the United States longer than legally permitted, according to DHS’s 2015 entry and exit overstay report.

Of those who did not leave the United States on time, around 482,781 are believed to still be illegally residing in the United States, according to the report, which was issued by DHS amid debate in Congress over an Obama administration initiative to permit around 170,000 new immigrants from Muslim-majority nations in 2016.

Senior Obama administration officials had trouble in the past year informing Congress about the number of individuals who had overstayed their visas. The administration also could not provide Congress with statistics about the number of Syrian refugees who had been granted residency in the United States in 2015.

Concerns about visa overstays come as Congress is investigating a sharp uptick in the number of terror plots hatched by foreign-born individuals legally granted entrance to the United States. Lawmakers estimate that at least 113 foreign-born individuals have been implicated in domestic terror plots since 2014.

Last year there were 219 illegal overstays from Afghanistan, 681 from Iraq, 564 from Iran, 56 from Libya, 1,435 from Pakistan, 440 from Syria, and 219 from Yemen, according to the report. Many are still in the United States illegally.

The statistics also do not encompass land ports, meaning those numbers compiled from Latin American could be higher as result of these types of border crossings.

The 527,127 individuals found by DHS to have overstayed their visas account for 1.17 percent of the "44,928,381 nonimmigrant admissions to the United States for business or pleasure" in 2015, according to the report.

"There were 482,781 Suspected In-Country Overstays," according to DHS. "The overall Suspected In-Country Overstay rate for this scope of travelers is 1.07 percent of the expected departures."

Additionally, 0.65 percent of the 20,974,390 individuals granted access to the United States under the visa waver program, which facilitates travel to America from 35 partner countries, are suspected of overstaying their authorization, according to the report.

When it comes to overstays from countries that do not participate in the visa waiver program, around 1.6 percent of the 13,182,807 individuals required to leave did not do so, the report states.

DHS has designated these visa overstay issues as "important for national security, public safety, immigration enforcement, and immigration benefit application processing," according to the report.

The foreign-born population in the United States stands at an all-time high of nearly 43 million people.

The Senate Immigration Subcommittee is schedule to hold hearing Wednesday to investigate this issue. Testimony will be provided from officials from U.S. Customs and Border Protection, as well as U.S. Immigration and Customs Enforcement.

Lawmakers on the committee remain concerned that the Obama administration has implemented a de facto policy of exempting visa overstays from its enforcement regime.

"By not enforcing visa overstays, the administration has flung the border open—millions get temp visas and then freely violate their entry contracts and shred their eviction notices," the subcommittee said in a statement. "Further, DHS has refused to complete the legally required biometric tracking system."