The 23-year-old Ukrainian model who was with slain opposition leader Boris Nemtsov tearfully recounted Monday their last dinner in a chic Red Square restaurant and their walk onto a nearby bridge — but said she did not see the gunman who pulled the trigger.

The emotional account by Anna Duritskaya came amid a swirl of speculation about who was responsible for the high-profile assassination and what it means for Russia.

While state-run and Kremlin-controlled media focused on a theory that the killing was a provocation aimed at staining President Vladimir Putin, his critics are holding the Russian leader responsible for creating an atmosphere that encouraged the crime by fanning nationalist, anti-Western sentiments and vilifying the opposition.

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Duritskaya said she has been questioned extensively by authorities. The Ukrainian Foreign Ministry said she has flown home to her native Ukraine, the Interfax news agency reported late Monday.

In her first public comments since the killing, Duritskaya said in an interview with Russia's independent Dozhd television that she waited for Nemtsov to meet her Friday night at the Bosco Cafe, a pricey restaurant in the former GUM department store on Red Square. He had just given a radio interview in which he had slammed Putin's "mad, aggressive policy" on Ukraine.

They dined and then walked across a bridge near St. Basil's Cathedral, heading for Nemtsov's apartment across the Moscow River from the Kremlin, she said, her eyes welling with tears.

Duritskaya said didn't see the man who shot Nemtsov, only a car speeding up.

"I don't know where he came from, but he was behind," she said of the gunman. "I didn't see the man. I turned round and all I saw was a light-colored car. I saw neither the brand nor the license plate of the car that was driving away."

After Nemtsov was shot, she saw a snowplow approaching them on the bridge and she said she asked its driver how to call police. The driver gave her the number, then drove away, she added.

TV Center, a station controlled by the Moscow city government, broadcast a poor-resolution video from a web camera that it said showed Nemtsov and Duritskaya shortly before he was killed. A vehicle that TVC identified as a snowplow moved slowly behind the couple, obscuring the view of the shooting. TV Center then circled what it said was the suspected killer jumping into a passing car.

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The video, which could not be independently verified, contradicted the initial statement by police, who said Nemtsov was shot from a passing car.

Investigators said they are looking into several possible links for Nemtsov's slaying, including an attempt to destabilize the state, Islamic extremism, the Ukraine conflict and his personal life. They have offered a reward of 3 million rubles (nearly $50,000) for any information.

Tens of thousands of supporters marched Sunday through central Moscow in a silent tribute to Nemtsov. Others mourned him in St. Petersburg and other European cities.

Putin quickly sent condolences to Nemtsov's 86-year old mother, promising her that the perpetrators of the "vile and cynical murder" will be brought to justice.

State television stations focused on allegations that Russia's enemies could be behind the hit, following comment by Putin's spokesman that the president saw the attack as a "provocation" aimed to destabilize the country. Chechnya's Moscow-backed strongman, Ramzan Kadyrov, openly blamed Western special services.

Kremlin critics pointed out that the site of the killing is one of the most heavily secured parts of the Russian capital, packed with police, plainclothes agents and security cameras.

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"The choice of place in front of the Kremlin ... points at the perpetrator's sense of impunity and his link to law enforcement structures," opposition lawmaker Ilya Ponomaryov wrote in his blog. "It was made for a picture: a prominent Russian opposition leader lying dead with the Kremlin stars and St. Basil's Cathedral as the backdrop."

Many commentators said that like other key opposition leaders, Nemtsov was constantly shadowed by police, so it would be hard to imagine his killing could go unnoticed by them. Some noted that Nemtsov died on the newly established holiday commemorating the Special Operations Forces, honouring troops who swept through Ukraine's Crimean Peninsula, setting stage for its annexation by Russia a year ago.

Nemtsov's killing was the biggest political assassination in Russia since another Kremlin foe, journalist Anna Politkovskaya, was shot to death in the elevator of her Moscow apartment building on Putin's birthday in 2006. Five Chechens were convicted in the case last year, but it has remained unclear who ordered the killing.

Some observers speculated that certain members of a hawkish, isolationist wing of the government could have had a hand in Nemtsov's death, possibly counting on it to provoke outrage abroad and further strain Russia's ties with the West. Those relations already are at their lowest point since the Cold War because of the Ukrainian crisis.

"Russia is at crossroads," Irina Khakamada, who once co-led a liberal party with Nemtsov, said on RBC TV. "Those who did it favoured a more aggressive path. It was a demonstration of their omnipotence."

Western officials have called for Russia to conduct a prompt, thorough, transparent and credible investigation into the slaying. Putin has ordered law enforcement chiefs to personally oversee the probe.

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White House spokesman Josh Earnest on Monday described Nemtsov as a "tireless advocate for his country, an opponent of corruption, and an advocate for human rights and greater transparency."

The business newspaper Kommersant quoted anonymous sources in the Interior Ministry as saying there was no closed-circuit TV video of the killing because the cameras in question were not working at the time.

However, Yelena Novikova, a spokeswoman for Moscow's information technology department, which oversees the city's surveillance cameras, said all cameras "belonging to the city" were operating properly when Nemtsov was killed. She said federal authorities also had surveillance cameras near the Kremlin that are not under her organization's control.

Novikova would not confirm the existence of any video of the killing, saying the police investigation was still underway.

Duritskaya's mother, Inna, who spoke to The Associated Press in the Ukrainian capital of Kyiv, said she worried that Russian authorities were "putting psychological pressure" on her daughter.

"I am worried that they want to use her, as a citizen of Ukraine, use her as though this situation is somehow something to do with Ukraine," she said Monday. "She has absolutely nothing to do with this killing. She is innocent. Stop abusing my child."