Nigerian President Muhammadu Buhari's visit to Washington, D.C., Monday to discuss the fight against Boko Haram has prompted critics to again question why Hillary Clinton refused to label the West African insurgents as terrorists during her State Department tenure.

Buhari and President Obama discussed U.S. support of Nigerian counterterrorism efforts, which was made possible by Secretary of State John Kerry's decision to place Boko Haram on the terrorist watch list in late 2013, just months after Clinton left office.

The State Department under Clinton resisted congressional calls to designate Boko Haram as a terrorist organization.

Robert Jackson, acting assistant secretary of state for African affairs, testified before Congress in May of last year that the agency could have acted sooner on Boko Haram.

Clinton's ties to Gilbert Chagoury, a prominent Nigerian businessman and Clinton Foundation supporter, have prompted at least one member of Congress to question the motives behind her decision not to slap Boko Haram with a Foreign Terrorist Organization classification.

Sen. David Vitter, R-La., wrote a letter to Kerry in March asking the State Department to turn over emails in which Clinton discussed Boko Haram.

"[G]iven the drastic foothold Boko Haram was allowed to gain prior to being designated an FTO, the nexus between the Department's decision against designating Boko Haram as an FTO and connections to outside groups should be brought forward," Vitter wrote.

Vitter questioned whether the Clinton's relationship with Chagoury influenced her decision against labeling Boko Haram a terrorist group.

He noted Bill Clinton had participated in events with Chagoury while his wife was secretary of state, and that the Nigerian land developer had "previously agreed to a $66 million plea deal during international investigation into corruption charges against him."

The Clinton Foundation's acceptance of donations from Nigerian companies has raised red flags in the past.

For example, the First Bank of Nigeria, one of the country's largest financial institutions, gave as much as $25,000 to the Clinton Foundation.

The son of the former chairman of the First Bank of Nigeria was sentenced to life in prison after attempting to blow up a Detroit-bound passenger flight on Christmas Day 2009.

Umar Farouk Abdulmutallab, Alhaji Mutallab's son, was dubbed the "underwear bomber" after authorities discovered explosives hidden in his underwear.

Bill Clinton accepted between $500,000 and $1 million from a Nigerian newspaper publisher in 2012 to speak at an event in Lagos.

The money went straight into the Clinton Foundation, donor records show.

Procter & Gamble's Nigeria operation won a prestigious State Department award in 2011. The company donated between $1 million and $5 million to the Clinton Foundation.

Hillary Clinton personally presented the company with the award at a ceremony in Washington, D.C.

Nestle Nigeria, a food manufacturing company, also donated to the Clinton Foundation.

Hillary Clinton reportedly invited Buhari to meet with her in 2009, years before the Nigerian president assumed office.

Powerful oil corporations have played a role in shaping Nigeria's policy. Many of the energy conglomerates have also enjoyed close ties to the Clintons.

Royal Dutch Shell, a major oil company and Clinton Foundation donor, reportedly infiltrated the Nigerian government in its efforts to dominate the energy sector there.

Leaked diplomatic cables suggest State Department officials received intelligence updates from Shell executives in Nigeria and were concerned about the effects of the deteriorating security situation on the country's oil industry.

The Nigerian ambassador under Hillary Clinton met with executives from Shell and other corporations — including fellow foundation donors Chevron, Schlumberger and ExxonMobil — in 2009 and advised them on ways to lobby the Nigerian government in their favor.

One of the reasons Hillary Clinton's officials cited for refusing to label the group as terrorists was "the possibility that doing so might heighten threats against U.S. and Western interests," according to CNN.