Congress is investigating how a former Iraqi insurgent who entered America as a refugee under a false name managed to get past the country's vetting system.

Early this week the Senate Homeland Security and Governmental Affairs Committee's Chairman Senator Ron Johnson (R-WI) sent a letter to Attorney General Jeff Sessions asking why government officials put off arresting the man for visa fraud after learning his identity.

"When [the Joint Terrorism Task Force] and the US Attorney's office for the Western District of Texas sought to prosecute this refugee, the local law enforcement and prosecutors allegedly 'met resistance' from officials within the National Security Division's Counter Terrorism section in Washington DC," Johnson wrote in a letter to Attorney General Sessions on 6 March. The letter's contents were discovered by Fox News.

"The 'resistance' allegedly occurred a few weeks before the 2016 election, and local authorities believed the lack of progress in this case was handled inadequately," Johnson continued.

The US Department of Justice and Johnson's office did not respond to a IBTimes UK's request for comment on the case and confirmation of the letter and its contents.

This week US President Donald Trump signed a new executive order to replace another on 27 January that aimed to block refugees from seven predominantly Muslim nations, including Iran, Libya, Somalia, Sudan, Syria, Yemen, and Iraq from entering the US for at least four months.

Under the new order, however, Iraqi refugees are being let in to the US because "Iraq is an important ally to defeat Isis," Secretary of State Rex Tillerson said during the new ban's roll out on Tuesday 7 March.

Tillerson said that an "intense review over the past month identified multiple security measures" that were being taken by the Iraqi government and the US State Department to vet Iraqi travellers and refugees.

The Trump administration has argued in federal court that the ban is necessary to review the vetting done on travellers and refugees from those nations to ensure that terrorists or criminals are not able to enter the US.

The Iraqi man under investigation entered the US under a false name, and while it is not known when he came to the US, it was through the refugee programme. All refugees currently undergo a 20 step series of interviews and vetting procedures through the United Nations, US State Department, United States Citizenship and Immigration Services, and the Department of Homeland Security.

The Joint Terrorism Task Force (JTTF) gained confirmation that the man claimed to have attacked American troops as an insurgent during the US occupation of Iraq during the second Iraq War, according to Fox News.

The man's behaviour in the US sparked an investigation by the JTTF, which was looking to press visa fraud charges against him.

Iraqi insurgents walk past a blazing vehicle that was carrying supplies to US forces after it was destroyed in the Baghdad suburb of Abu Ghraib, 8 April 2004 Reuters/Imad Aqrawi