A Republican senator has accused the US government of mishandling the case of a known arms trafficker of the jihadist group Islamic State who was also linked to the murder of four Americans in Benghazi, Libya, three years ago.

Senator Chuck Grassley of Iowa recently wrote to Federal Bureau of Investigation (FBI) Director James Comey and US Attorney General Loretta Lynch questioning why Ali Awni al-Harzi "somehow slipped out of our government's reach."

Al-Harzi was a Tunisian recruiter and an ISIS member killed last month during a US airstrike in Iraq.

Grassley pointed out that the FBI was able to interrogate Al-Harzi in Tunisia before he was released in January 2013, or months after the Sept. 11, 2012 attack on the US diplomatic mission in Benghazi.

"Although Al-Harzi was on our radar in 2012 for his terrorist activities, he somehow slipped out of our government's reach, only to continue his terrorist career for years," Grassley wrote.

Grassley noted in his letter that the US government, particularly former State Department Secretary Hillary Clinton, consented to Al-Harzi's release after Tunisia's government gave an assurance that his movements will be closely monitored.

Tunisian authorities, however, eventually lost track of Al-Harzi, allowing him to join the ISIS.

The Republican senator also questioned why the American government failed to extradited Al-Harzi to the United States despite the wealth of evidence against him, including his own social media posts about the Benghazi attack and classified documents kept by the FBI.

"This raises important questions about the Obama administration's policies and procedures related to the apprehension, interrogation, and detention of terrorists and the roles of the Justice Department and the FBI," the senator said in his letter.

Grassley also asked why the US government never considered a military operation to retrieve Al-Harzi.