MOSCOW — The assassins come with pistols, hammers and knives. They ambush cars or plant bombs. The hunted: Chechen bloggers critical of Russian President Vladimir Putin's protege, Chechen leader Ramzan Kadyrov, or Chechen rebel veterans who fought for independence from Russia.

The latest victim, 43-year-old Mamikhan Umarov, who posted frequent criticism of Kadyrov on YouTube, was gunned down outside a shopping center in the Austrian town of Gerasdorf north of Vienna late Saturday. A 47-year-old Russian fled by car, was tracked by a police helicopter and was arrested about 100 miles from the scene of the crime. A second person — who arrived with the victim at the crime scene, according to Austrian authorities — was also arrested.

Austrian counterterrorism police are investigating the crime. The police directorate said Umarov had declined police protection before he was killed and that it was unclear whether the crime had a political motive.

The murder follows a long string of assassinations and attacks on Chechen exiles in Europe, Turkey and the Middle East since 2004, when a former acting leader of Chechnya, Zelimkhan Yandarbiyev, was assassinated in a car-bomb attack in Doha, the capital of Qatar.

Saturday's attack was not the first killing of a Chechen in Austria. In January 2009, one of Kadyrov's former bodyguards, Umar Israilov, was killed in broad daylight in Vienna.

Some of the attacks, including assassinations of former Chechen rebel fighters, have been linked to Russian security agencies. Others have targeted exiled journalists and critics of Kadyrov, who is accused by human rights advocates of abuses against political opponents, activists, LGBT people and others. He denies abuses or ordering assassinations.

However, Putin has given Kadyrov a free hand to do whatever he deems necessary to stabilize Chechnya, where rebels fought two grueling independence campaigns against Moscow in the 1990s and early 2000s.

The assassinations of Chechens in exile appear to be part of a broader pattern of brazen attacks by Russians in foreign countries — including the 2018 poisoning in Britain of former Russian military intelligence officer Sergei Skripal and his daughter Yulia using the nerve agent Novichok, and the fatal 2006 poisoning of another former Russian spy, Alexander Litvinenko, using a radioactive agent, polonium-210.

According to British authorities, the Russian military intelligence agency GRU was responsible for the Skripal attack. In June, Germany accused Russia of the assassination of a former Chechen commander, Zelimkhan Khangoshvili, 40, in the Kleiner Tiergarten park in Berlin last year.

The killer rode up on a bicycle and shot Khangoshvili, a citizen of Georgia, several times in the head.

An online investigative unit, Bellingcat, reported that a secretive unit within Russia’s Federal Security Service (FSB), a successor agency to the KGB, carried out Khangoshvili’s assassination. It identified the assassin as a ­54-year-old Russian contract killer and said the killing was arranged by the FSB’s special counterterrorism unit Department V. German authorities arrested the suspect minutes after the assassination, and he remains in custody.

Putin called Khangoshvili a bandit but denied that Russia played any role in his death. At a December summit meeting on the war in eastern Ukraine at which German Chancellor Angela Merkel was also present, Putin called Khangoshvili “cruel and bloodthirsty” and claimed the former commander took part in an attack that killed 98 people and in a 2004 bombing of Moscow’s Metro. Kremlin spokesman Dmitry Peskov said Germany’s decision to expel two Russian diplomats in connection with the killing was “absolutely unfounded.” Russia expelled two German diplomats in retaliation.

German authorities indicted the Russian suspect last month in Khangoshvili’s murder.

Bellingcat last week reported that a second Russian agent was involved in the Berlin assassination and linked that person to a group of FSB agents believed responsible for the killing of another Chechen in Istanbul in 2015.

In that incident, Abdulvakhid Edilgeriev, a militant connected with the Chechen rebellion against Moscow, was in his car with his 4-year-old niece when another car rammed him at speed from behind. He fled but was pursued and shot and stabbed in the street. Turkey later arrested two Russian suspects — Yuri Anisimov, 52, and Alexander Smirnov, 55 — and handed them over to Moscow.

At least seven other Chechens were assassinated in Turkey between 2003 and 2013, according to the Jamestown Foundation, and several others have survived assassination attempts.

In recent months, several more prominent Chechens have been attacked in Europe. In one case, a critic of Kadyrov, Chechen emigre Imran Aliyev, was found dead with more than 130 stab wounds in the Coq Hardi hotel in Lille, France, in January. French prosecutors said a male suspect departed for Russia the day after the killing.

Weeks later in the town of Gavle, Sweden, a man with a hammer came for another prominent blogger critical of Kadyrov, Tumso Abdurakhmanov, whose YouTube channel has more than 350,000 subscribers. Abdurakhmanov had blamed Kadyrov for the Lille killing.

He was sleeping in his house when he was struck by hammer blows to the head. He managed to fight off and subdue the attacker and streamed a video of himself panting and standing over the man, asking him: “Who sent you? Where have you come from?”

“From Moscow,” the man replied.

“How did you find out my address?”

“They told me,” the attacker said, adding, “They have my mother.”

The attack came after Magomed Daudov, speaker of the Chechen parliament and a close Kadyrov ally, announced a blood feud against Abdurakhmanov and vowed to find him. He was angered over a phone interview in which the blogger strongly criticized him.

In Chechnya, Abdurakhmanov’s family had disowned him on local media, with one uncle saying, “If anyone kills him, we allow that person to take his blood.”

Swedish authorities later detained Ruslan Mamaev, a 29-year-old Russian man suspected of the attack, and a 30-year-old Russian woman, said to be his accomplice. Chechen authorities denied involvement in the attack, which took place on Daudov’s birthday.

Before Saturday’s killing near Vienna, Abdurakhmanov drew attention to what he called abuses by Chechen authorities against the family of Umarov, the Chechen critic who was gunned down Saturday.

“This is a man whose brother was kicked in front of the video camera and killed by them. His mother was brought and dragged by the hair to force her to reveal her son’s whereabouts,” he posted on YouTube on May 31.

In October 2017, former Chechen fighter Amina Okuyeva, 34, was killed when gunmen ambushed her and her husband in their car near a railway crossing outside the Ukrainian capital, Kyiv. Her husband, Adam Osmayev, who had been accused by Russian authorities of plotting to kill Putin, escaped. Another assassination attempt against him earlier in 2017 failed.

Ukrainian authorities said they suspected that either Russian agents or pro-Kremlin Chechen assassins carried out the attack. Osmayev took part in the war in eastern Ukraine against Russian-backed separatists.

The previous month, a car bomb in central Kyiv killed Ali Timayev, a Chechen fighter with Georgian citizenship.

Some of the Chechen assassination victims have been wanted by Russia’s FSB for years. According to Bellingcat, in 2012 the FSB shared with German intelligence a wanted list including 19 Chechens, five of whom have since been assassinated.