A Chechen man accused of plotting to kill Russian President Vladimir Putin was wounded and his wife killed when their car was ambushed near Ukraine's capital, Kiev.
Adam Osmayev, wounded in one leg, said his wife Amina Okuyeva was shot in the head. A hail of bullets hit their car in Hlevaha village, a rural area.
Ukrainian officials suspect that the Russian state was involved.
Okuyeva has hero status in Ukraine for having fought the pro-Russian rebels.
Speaking to Ukrainian TV in hospital Mr Osmayev said: "She was shot in the head. I drove as much as I could until the car stopped, I don't know, the engine was also hit. I tried to give her first aid, but she was shot in the head."
It is the second time this year that the couple have been assassination targets. In June, a gunman posing as a French journalist opened fire at Mr Osmayev in Kiev, but Ms Okuyeva fired back, wounding the man, who was reported to be a Chechen criminal.
In 2012, Russian officials said Mr Osmayev was suspected of being part of a plot by Islamists to kill Mr Putin.
Russian media reported at the time that the plotters were planning to plant mines on Kutuzovsky Avenue in Moscow, used by Mr Putin on a daily basis.
Russia later asked Ukraine to hand over Mr Osmayev - but the Kiev authorities refused to do so, saying they wanted to wait until the European Court of Human Rights considered his appeal against extradition. He was freed after two-and-a-half years in a Ukrainian prison.
More on Ukraine conflict
Warning of 'harsh response'
The Chechen couple are well-known figures in Ukraine, after fighting as volunteers for Ukraine's military in the Donbas region against pro-Russian rebels.
Mr Osmayev commanded the Dzhokhar Dudayev volunteer battalion, named after the late Chechen rebel leader who led a nationalist uprising against Russian forces in the 1990s.
Ms Okuyeva served as a medic in Ukraine's Kiev-2 battalion and fought in Debaltseve, taken by the pro-Russian rebels in February 2015 after months of clashes.
On Facebook the head of Ukraine's National Security Council, Oleksandr Turchynov, wrote that "Russia, continuing its aggression in eastern Ukraine, has unleashed terror deep behind the lines, killing courageous defenders of our country".
The killing of Ms Okuyeva "is a challenge to our country which requires a harsh and suitable response", he wrote.
Ukrainian Prime Minister Volodymyr Groysman wrote on Facebook: "Let the true patriot of Ukraine be remembered forever!"
No-one has so far claimed responsibility for Monday's attack.
It comes less than a week after a Ukrainian nationalist MP, Ihor Mosiychuk, was injured in a vehicle bombing in Kiev.
One of his bodyguards and another person were killed in the blast. Ms Okuyeva had once worked for him as an adviser.
Kiev has in recent years seen a number of deadly attacks on high-profile politicians and journalists.
Low-level skirmishes continue in eastern Ukraine, where Kiev's forces face heavily armed separatists who control a large swathe of Donetsk and Luhansk regions. The Russian-speaking rebels have sworn loyalty to Moscow.