TRIPOLI (Reuters) – Gunmen shot dead an American teacher working in the Libyan city of Benghazi on Thursday more than a year after Islamist militants stormed the U.S. consulate there, killing the U.S. ambassador and three others.

Libya’s fragile government is struggling to contain former fighters and militants who two years after the fall of Muammar Gaddafi are challenging a fragile state that is still building up a national army with Western aid.

Security sources and school officials said the American was shot as he was exercising in the city where he worked as a chemistry teacher at an international school.

“He was doing his morning exercise when gunmen just shot him. I don’t know why,” said Adel al Mansouri, a manager at the school in Benghazi. “He was so sweet with everyone.”

U.S. State Department spokeswoman Marie Harf confirmed that a U.S. citizen was shot and killed in Benghazi but gave no further details. The University of Texas released a statement identifying the teacher as Ronnie Smith, who had received a master’s degree in chemistry from the school.

It was not immediately clear who was responsible for the attack but assassinations and bombings are common in Benghazi. Three soldiers also were killed in attacks there on Thursday.

White House spokesman Jay Carney said there had been claim of responsibility for the killing and that the Obama administration expected the Libyan government to investigate thoroughly.

Libyan special forces there have been battling militants from Ansar al-Sharia, the group U.S. officials blame for attacking the American consulate in the eastern city in September 2012.

Late last month fighting broke out between army special forces and members of Ansar al-Sharia in Benghazi, killing at least nine people before the Islamist militants retreated from their main base in Benghazi.

Worried about stability in the key European oil supplier, Western governments are training up Libya’s nascent armed forces but those efforts are just starting and militias still control parts of the country.

Former fighters once employed to guard oil sites have taken over ports in the east, disrupting exports in protests for regional autonomy while protesters in the west blocked gas pipelines and ports to demand more ethnic rights.

(Reporting by Ayman al-Warfalli in Benghazi; Feras Bosalum in Tripoli; Writing by Patrick Markey; Editing by Mark Trevelyan and Bill Trott)