A local woman has been arrested in the United Arab Emirates, suspected of murdering an American teacher in a toilet in an Abu Dhabi shopping centre.
Police said the suspect had also tried to place a bomb outside the home of an American doctor.
The stabbing to death of the teacher, identified by US media as Ibolya Ryan, on Monday shocked the UAE.
The attacker, wearing a full black veil, was captured on CCTV entering and leaving the centre.
Interior Minister Saif Bin Zayed said: "We are witnessing an unprecedented heinous crime in the UAE."
Col Rashid Borshid, of Abu Dhabi's Criminal Investigation Department, said the suspect had also tried to place a makeshift bomb outside the home of the American expatriate doctor. The device was made safe.
Col Borshid said the question of a terrorist attack was "under investigation now, and I cannot say yes or no".
ABC News and other US media said Ms Ryan was the 47-year-old mother of 11-year-old twins. Her identity has not been made public officially.
The Associated Press news agency quoted the company that placed the teacher in the job, Footprints Recruiting, as saying: "We await the results of the ongoing police investigation before making conclusions about why this senseless tragedy occurred.
"We are confident that the UAE in general and Abu Dhabi in particular remains a safe environment in which to live and work."
Police said the teacher was stabbed with a "sharp tool" after an argument in the toilet in the shopping centre on Reem Island.
CCTV footage showed the suspect walking towards the toilet. About 90 minutes later, the suspect appears again in the lobby, brushing off a woman who tries to stop her, and flees up an escalator. A trail of blood is also seen.
The UAE has a large population of Western expatriates and sees itself as a safe haven in the Middle East.