Sens. David Vitter and Chuck Grassley pressed the Justice Department Tuesday on why the State Department, under the leadership of Hillary Clinton, refused to designate Boko Haram as a terrorist organization.

"For two years, Boko Haram grew as Secretary Clinton and her Department of State refused official requests to designate Boko Haram" as a terror group, the Republican lawmakers wrote in a letter to the Attorney General Loretta Lynch on Tuesday.

Vitter and Grassley were referring to a period between 2011 and 2012, during which the Justice Department joined other federal agencies in recommending the designation for the Nigerian terror outfit. The State Department did not grant one until the early months of Secretary John Kerry's tenure in 2013.

Critics have questioned the motives behind State Department officials' reluctance to put Boko Haram on the list of official terror organizations under Clinton.

Vitter has in the past raised the possibility that Clinton's proximity to Gilbert Chagoury, one of Nigeria's most prominent businessmen, was related to the delay. The agency has stonewalled attempts to request records about Chagoury's possible connections to the decision-making process.

Justice Department officials were among those urging Clinton's State Department to label Boko Haram a terror organization, particularly after the group claimed responsibility for a 2011 bombing at a United Nations building in the Nigerian capital.

Vitter's office noted that in 2014, the Senate Armed Services Committee "uncovered evidence that suggests the State Department deliberately downplayed the dangers of Boko Haram and attempted to obfuscate the facts behind the 2011 UN bombing in order to avoid having to make the designation." His staff pointed to unnamed "Clinton Foundation employees working for the State Department" who may have also been "directly involved in the non-designation [of Boko Haram] for the two years the Department refused to make the designation."

In their letter to Lynch, Vitter and Grassley cast doubt on the State Department's past argument that the Nigerian government opposed the U.S. counterterrorism activity in Nigeria that would have accompanied a terror designation.

"Nigerian President Goodluck Jonathan had requested increased counterterrorism cooperation during a June 2011 meeting with President Obama," the GOP lawmakers wrote, citing documents obtained through the Freedom of Information Act. "This information raises questions about the Department of State's claims that the Nigerian government opposed increased U.S. counterterrorism efforts in Nigeria."

Vitter has for more than a year pressed administration officials for more information about the protracted period in which Boko Haram mounted attacks on Western targets but was left off the official Foreign Terror Organizations list.

In a March 2015 letter to the State Department, the Louisiana Republican made clear he suspected a link between the agency and the Clinton Foundation had played a role in the foot-dragging that prevented Boko Haram from receiving a terror designation until 2013.

"Disturbingly, while the [State] Department was refusing to make the FTO designation, former President Bill Clinton, also the head of the Clinton Foundation and account owner of the Clinton email domain, participated in events with a long-time donor and major Nigerian land developer, Gilbert Chagoury, who previously agreed to a $66 million plea deal during international investigation into corruption charges against him," Vitter wrote in a letter last year. "We need to know if Mr. Chagoury had any influence in the decision not to designate Boko Haram an FTO, or had any other influence with Sec. Clinton's foreign policy decisions."

Chagoury is just one of many foundation donors whose relationship with the former secretary of state has presented the Clinton campaign with uncomfortable questions.

Emails made public last month suggest Chagoury's requests for access to high-level officials at the State Department in 2009 were facilitated by a top Clinton Foundation executive.

In their letter to Lynch on Tuesday, Vitter and Grassley asked the Justice Department for copies of the formal requests it made for a terror designation of Boko Haram before 2013.