It's a tough time to be a Ukraine-based critic of Russian President Vladimir Putin.

On Tuesday, Amina Okuyeva, the wife of prominent Putin critic, Adam Osmayev, was shot and killed in an ambush just outside Kiev. Osmayev himself, whom Russia accuses of being involved in a 2012 assassination plot against Putin, was wounded but is expected to recover. (The 2012 plot against Putin was detected by Ukrainian authorities when the terrorists prematurely detonated their explosives.)

Tuesday's attack seems to have been the follow up to a June assassination attempt targeting the couple. In that earlier incident, a gunman pretending to be a journalist attacked the couple but was shot and killed by Okuyeva. According to Ukrainian authorities, the gunman was a Russian national in possession of a Ukrainian passport. The implication: that the would-be killer was a Russian intelligence operative or agent.

Regardless, this latest attack is just the tip of the iceberg.

Last week, a Ukrainian nationalist member of parliament, Ihor Mosiychuk, was targeted with a bomb as he left a TV studio. A number of senior Ukrainian military and intelligence officers have also been assassinated this year. What unifies these targeted individuals is their criticism or obstruction of the Russian government and/or Chechen leader, Ramzan Kadyrov.

Yet while Kadyrov is a particularly aggressive individual, he is also Putin's guy. As I've explained, Kadyrov's penchant for unpredictable behavior and avowed brutality gives Putin a semi-deniable intermediary with which to achieve his interests. Correspondingly, even if Kadyrov's people are behind these attacks, they're very likely operating with Putin's consent.

Ultimately, more attacks are probably coming: the lesson of Russian assassination plots is that they don't stop until a government pushes back. And unfortunately, for politically dysfunctional Ukraine, challenging Russia's invasion of its south-eastern provinces is hard enough.