There is growing concern on Capitol Hill that a recent spike in the number of foreigners arrested in the United States on terrorism charges is the result of a flawed Obama administration immigration policy that will grant residence to around 170,000 individuals from Muslim-majority nations in the coming year, according to conversations with lawmakers and congressional sources.

With the indictment last week of two Iraqi refugees who allegedly conspired with ISIS, a number of lawmakers are now warning that poor screening methods and an administration plan to boost immigration will endanger national security.

Recent efforts by Congress to investigate the immigration histories of at least 113 foreign-born individuals snagged on terror charges since 2014 have been blocked by the Obama administration. There are at least 1,000 open investigations into ISIS members residing in the United States, sources said.

New cases of legal immigrants participating in alleged terrorist plots are cropping up on a near daily basis, according to Sen. Ted Cruz (R., Texas), who has been spearheading an effort to investigate how these individuals are passing through U.S. security screenings.

"Almost every day there is a new case that documents the relentless attacks being waged against our nation," said Cruz. "We have militants who have infiltrated waves of refugees to come to American and plot attacks. We have Americans who have travelled abroad to train returning to wage jihad here. We have home-grown terrorists who are receiving instruction online to hatch attacks."

"We have to stop treating this pattern as the isolated actions of lone wolves," Cruz added. "They are a pack."

Networks of radicalized individuals are already forming within the United States due to a massive influx of more than 680,000 immigrants from Muslim-majority nations during the past five years, according to Stephen Miller, spokesman for Sen. Jeff Sessions (R., Ala.), another lawmaker who has worked to expose flaws in the U.S. immigration system.

"Unprecedented rates of migration, combined with poor screening, have created a perfect storm," Miller said. "There are now nearly 1,000 open ISIS investigations within our borders, and underneath these cases are large extremists networks made possible by a total lack of immigration controls."

On Tuesday, a Maryland man was arrested for attempting to join the Al Shabaab terror network. Last week, a Philadelphia police officer was nearly killed by an assailant who pledged allegiance to ISIS. Late last year, two radicalized immigrants went on a massive shooting spree in San Bernardino, California, in what was the biggest terror attack on U.S. soil since 9/11.

Meanwhile, previously undisclosed documents released by the State Department on Wednesday reveal that it has known since 2004 that "Arab extremists" are being smuggled into the U.S. via Mexico.

The administration and majority of Congress have passed on new pieces of legislation aimed of halting immigration from countries where major terror groups are based.

"We have to coordinate our actions to protect ourselves—and since President Obama told us last night [at the State of the Union] he has no intention of taking robust action to defeat this enemy, Congress must act by passing TRIPA," a piece of legislation that would place a three-year hold on granting residency in the United States for refugees coming from nations where al Qaeda and ISIS control significant territory, Cruz said.

He also called for the passage of another stalled piece of legislation that would cancel the U.S. citizenship of any person who travels abroad to join a terror group.

Cultural concerns about the way terrorism is discussed also has hampered America’s ability to address the growing threat, according to Cruz, who called for "ending the self-defeating political correctness that prevents us from calling radical Islamic terrorist attacks for what they are."

Cruz and Sessions launched an investigation last week into the Obama administration’s refusal to release the immigration records of those 113 foreigners brought up on recent terrorism charges, including the San Bernardino shooters.

The administration, including the Departments of State and Homeland Security, has withheld this information from the senators since August.

Asked to address the delay this week, officials from State and Homeland Security both declined to provide information.

A Homeland Security source said that it would "respond directly to the senators," though the source would not specify when.

A State Department official confirmed that it had received notification that Cruz and Sessions had opened an investigation, but also declined to provide further information.

"We have received the letters and will respond appropriately," the official said.

Other prominent lawmakers expressed frustration that little is being done to address known flaws in the immigration and U.S. screening system.

"There is no question that the U.S. remains vulnerable to infiltration by radical Islamists on a wide variety of fronts," said Rep. Ron DeSantis (R., Fla.), a member of the House Committee on Foreign Affairs.

DeSantis, who also chairs the House’s Subcommittee on National Security, publicly vented his frustration about flaws in the U.S. screening process in late December, when he questioned a senior Homeland Security official in what was later seen as one of the administration’s more embarrassing moments on Capitol Hill.

That official, Kelli Ann Burriesci, the department’s deputy assistant secretary for screening coordination, was unable to tell Congress how many individuals with expired visas are still living in the United States. She also did not know how many Syrian refugees had entered the United States in the last year, nor the number of Americans who have travelled to Syria.

"The federal government doesn't control our borders and therefore cannot prevent illegal entry of those who want to do us harm," DeSantis said on Wednesday. "The refugee program cannot adequately screen refugees and so we've continued to see individuals admitted as refugees brought up on terror charges and will no doubt see many more."

At the same time, "the legal immigration system—including fiancé visas and visa waiver travel from countries such as Belgium that are teeming with jihadists—effectively provides a pathway for terrorists and their supporters to enter our country unimpeded," DeSantis warned." These disastrous policies are inviting more terror attacks against the United States."

With the immigration issue likely to take center stage in 2016 with the planned influx of up to 10,000 Syrian refugees, terrorism expert Patrick Poole noted that the threat of terror attacks in the United States is at the highest point in modern history.

"The ISIS-inspired domestic terror threat is unlike anything we've seen since 9/11," Poole said. "By the midpoint of 2015 we saw the number of international-related terror arrests eclipse the previous two years combined. That's simply staggering."

Major American cities have reported "nearly 100 active ISIS-related terror investigations," according to Poole, who explained that the influx has taxed local FBI bureaus.

"As we tragically saw in San Bernardino last month, the potentially lethal nature of the threat right now would be hard to overstate," Poole said.