The FBI mishandled the case of an Islamic State terrorist linked to the murder of four Americans in Benghazi, the chairman of the Senate Judiciary Committee says.

Sen. Chuck Grassley (R., Iowa) wrote a letter to James Comey, the FBI director, and Loretta Lynch, the U.S. attorney general, earlier this month regarding Ali Awni al Harzi, a Tunisian recruiter and arms trafficker for the Islamic State (IS, also known as ISIS or ISIL) who was killed in a U.S. drone strike last month.

Just after the Sept. 11, 2012, attack on the U.S. diplomatic mission in Benghazi began, al Harzi is reported to have posted messages on social media about the fighting, which assisted U.S. intelligence in later finding him in Turkey. He was then transferred to Tunisian authorities.

Once in Tunisian custody, he was eventually released in January 2013 after being interviewed for three hours by FBI investigators. He went on to join IS in Iraq.

Grassley asks in his letter how al Harzi "somehow slipped out of our government’s reach," despite his reported links to Ansar al-Sharia, an al Qaeda-affiliated group that is believed to be a key perpetrator of the Benghazi assault. Militants killed four Americans in the attack, including Ambassador Chris Stevens.

"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. "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."

FBI investigators were finally able to interview al Harzi in December 2012 after months of requests to Tunisian authorities. In addition to his social media postings, U.S. officials have said video footage captured al Harzi at the attack on the consulate in Benghazi and made him a "person of interest."

Yet a Tunisian judge released him in January 2013—a move opposed by the attorney general’s office in the country—citing a lack of evidence. According to Grassley’s letter, Hillary Clinton, U.S. secretary of state at the time, told lawmakers in congressional testimony that she was "assured that [al Harzi] is under the monitoring of the court" after his release. She also consulted Robert Mueller, then the FBI director, about his case.

"He was released, because at the time—and Director Mueller and I spoke about this at some length—there was not an ability for evidence to be presented yet that was capable of being presented in open court," Clinton said.

Tunisian authorities eventually lost track of al Harzi, who had joined the Islamic State, also known as ISIS or ISIL. Seth Jones, a terrorism expert at the RAND Corporation, told the New York Times that al Harzi’s "links with ISIL recruitment and financial networks in such locations as Europe, Africa and the Persian Gulf helped ISIL expand from a local group to a more globalized organization." U.S. forces relocated al Harzi last month in Mosul, Iraq, where he was killed in a drone strike.

Tom Joscelyn, a senior fellow at the Foundation for Defense of Democracies and an expert on jihadist networks in the Middle East, said that U.S. officials have never explained why they allowed al Harzi to walk free. Tunisian authorities did not appear to monitor al Harzi very closely after his release, he said, despite their assurances to the Obama administration. And U.S. officials apparently chose not to use a wealth of evidence at their disposal to further prosecute him, including social media posts, details from the FBI interview, or other classified information, he added.

"None of the excuses that were made for him being released, or for not being concerned about his release, made any sense," Joscelyn said.

In his letter, Grassley posed several questions to the FBI and Justice Department, including why al Harzi was not extradited to the United States, and why classified information was not used in a criminal case against him. He also asked whether officials considered a military operation to retrieve al Harzi.

U.S. special operations forces captured Ahmed Abu Khattala, leader of Ansar al-Sharia in Benghazi and the suspected coordinator of the 2012 attack, in a raid last year.

While questions remain unanswered about al Harzi’s release, the House Select Committee on Benghazi has been attempting to obtain all government records pertaining to the attack and then issue a final report.

Rep. Jim Jordan (R., Ohio), a member of the committee, said the State Department’s response to records requests has been "pathetic." According to a recent report, some emails between Clinton and her top aides—which involved the Obama administration’s talking points about the attack—were withheld from the committee due to "deliberative process privilege."

"We’re not going to have a complete record," Jordan said. "We’re just going to have do the best we can, and try to get the information for the American people and also importantly for the families of those four individuals" who were killed.

Neither the FBI nor the Justice Department responded to requests for comment for this article. A spokesman for Clinton also did not respond.

Mueller, the former FBI director, declined to comment through a spokeswoman.