Moscow (CNN) The Ukrainian girlfriend of slain Russian opposition leader Boris Nemtsov faces constant psychological pressure from Moscow authorities and fears she will be implicated in his death, her mother told CNN.

Model Anna Duritskaya, 23, was walking with Nemtsov on Friday night when a gunman shot the politician about 100 meters (330 feet) from the Kremlin.

"She was crying and she was saying Boris had been killed and he's lying next to me," said her mother, Inna Duritskaya. "She was in such shock she couldn't say anything else."

Duritskaya said her daughter was questioned until 2 a.m. Monday. She said she fears authorities will implicate her daughter in the death because they have threatened to take her attorney away from her.

The model had been staying at the apartment of an aide to Nemtsov under constant police guard.

Late Monday, Yevhen Perebyinis, a spokesman for Ukraine's Foreign Ministry, tweeted that Duritskaya had left Moscow for Kiev.

Surprising relationship

Photos: World reacts to Boris Nemtsov's killing Photos: World reacts to Boris Nemtsov's killing People follow the coffin of Russian opposition leader Boris Nemtsov during a farewell ceremony in Moscow on Tuesday, March 3. Nemtsov, a vocal opponent of Russian President Vladimir Putin's, was gunned down hundreds of feet away from the Kremlin on Friday, February 27. The assassination has spawned a flood of conspiracy theories; many suspect the Kremlin of either direct or indirect involvement. Putin condemned the killing and ordered three law enforcement agencies to investigate, the Kremlin said. Hide Caption 1 of 16 Photos: World reacts to Boris Nemtsov's killing Dina Eidman, the mother of Nemtsov, left, and other relatives pay their last respects in Moscow on March 3. Hide Caption 2 of 16 Photos: World reacts to Boris Nemtsov's killing People pay tribute to Nemtsov on Monday, March 2, at the scene of the opposition leader's shooting in Moscow. Hide Caption 3 of 16 Photos: World reacts to Boris Nemtsov's killing People march in memory of Nemtsov in Moscow on Sunday, March 1. Hide Caption 4 of 16 Photos: World reacts to Boris Nemtsov's killing A woman carries a sign with Nemtsov's image during a march in St. Petersburg, Russia, on March 1. Hide Caption 5 of 16 Photos: World reacts to Boris Nemtsov's killing A woman leaves flowers in Nemtsov's memory during a march in central Moscow on March 1. Hide Caption 6 of 16 Photos: World reacts to Boris Nemtsov's killing People march near the Kremlin in Moscow on March 1. Hide Caption 7 of 16 Photos: World reacts to Boris Nemtsov's killing Watched by police in boats on the Moskva River, crowds march across a bridge in Moscow on March 1. Hide Caption 8 of 16 Photos: World reacts to Boris Nemtsov's killing People gather to honor Nemtsov during a ceremony at Maidan, or Independence, Square in Kiev, Ukraine, on March 1. Hide Caption 9 of 16 Photos: World reacts to Boris Nemtsov's killing A woman places a candle at a makeshift memorial in front of the Russian Embassy in Vilnius, Lithuania, on March 1. Hide Caption 10 of 16 Photos: World reacts to Boris Nemtsov's killing People gather at Innocent Plaza in Paris on March 1 to remember Nemtsov. Hide Caption 11 of 16 Photos: World reacts to Boris Nemtsov's killing A man carries a sign with Nemtsov's image March 1 in Moscow. Hide Caption 12 of 16 Photos: World reacts to Boris Nemtsov's killing Marchers gather in central Moscow on March 1. Hide Caption 13 of 16 Photos: World reacts to Boris Nemtsov's killing People march near the Kremlin in Moscow on March 1. Hide Caption 14 of 16 Photos: World reacts to Boris Nemtsov's killing People light candles at a monument dedicated to political prisoners in St. Petersburg on Saturday, February 28. Hide Caption 15 of 16 Photos: World reacts to Boris Nemtsov's killing A group of European Union ambassadors to Russia lay flowers on February 28 at the site where Nemtsov was gunned down in Moscow. Hide Caption 16 of 16

The assassination has spawned a flood of conspiracy theories. Nemtsov was one of President Vladimir Putin's most outspoken critics, and many suspect the Kremlin of either direct or indirect involvement.

Inna Duritskaya said her daughter knew the risks of getting involved with the prominent opposition leader.

"Of course when I heard she was dating him it was a bit of a shock," she said.

"Then when I met him in person I liked him a lot as a person. He was straightforward, he treated my daughter very well, she was happy. And I couldn't influence it anyway; my daughter did what she wanted to. Of course, I was worried and scared, but I couldn't change it anyway."

The massive outpouring

Thousands of Nemtsov supporters gathered Sunday in Moscow to mourn the death of the 55-year-old, who was deputy prime minister during President Boris Yeltsin's administration.

JUST WATCHED Thousands mourn opposition leader Boris Nemtsov Replay More Videos ... MUST WATCH Thousands mourn opposition leader Boris Nemtsov 01:38

The event took place the same day Nemtsov was supposed to lead a rally in Moscow decrying Russia's policies on Ukraine.

Ukrainian President Petro Poroshenko said Saturday that Nemtsov was about to reveal information that would be damaging to Russian interests, Ukraine's state-run Ukrinform news agency reported.

"A few weeks ago I had a conversation with him on how to build relations between Ukraine and Russia, as we would like them to be," Ukrinform quoted Poroshenko as saying

"Boris said that he was going to make public the strong evidence for participation of the Russian armed forces in Ukraine. Someone was very afraid of that. Boris was not afraid, the executioners were afraid. They killed him."

The hunt for the killer

Putin condemned the killing and ordered three law enforcement agencies to investigate the shooting, the Kremlin said.

JUST WATCHED Surveillance video appears to show the Nemtsov killing Replay More Videos ... MUST WATCH Surveillance video appears to show the Nemtsov killing 02:10

The President also wrote to Nemtsov's mother, saying he shared her grief, and promised to bring those behind the killing to justice, the Kremlin said.

Police said they are looking for a man with short hair who stands between 5 feet 7 inches and 5 feet 9 inches, Russia's Sputnik news agency reported . The shooter was wearing blue jeans and a brown sweater.

The Investigative Committee of the Russian Federation is offering a reward of 3 million rubles ($49,000) for information leading to the arrest of the killer. It said the tipster will be guaranteed anonymity.

Duritskaya told the interviewer she did not see who killed Nemtsov, but that when she turned around she saw a light-colored car driving away. Duritskaya said that her wish was to go home to Ukraine to see her mother and that she did not believe she could attend Nemtsov's funeral on Tuesday.

The conspiracy theories

Speculation has pointed blame at everyone from Putin, to a Putin associate, to someone just irked by Nemtsov's business activities.

"I doubt it was a direct order from Putin, but it was this toxic atmosphere of hatred," Russian pro-democracy activist Garry Kasparov said. "It has been propagated by Russian television 24/7."

JUST WATCHED Garry Kasparov remembers his friend Boris Nemtsov Replay More Videos ... MUST WATCH Garry Kasparov remembers his friend Boris Nemtsov 01:37

Nemtsov's friend and political colleague Ilya Yashin blamed Putin more directly: "It's clearly a political murder," Yashin said. "I don't know who killed Boris, but I know that it's the government and personally Putin who are responsible for it."

Putin offered a different theory: that the killing was a contract hit, but one meant to fire up political strife, his spokesman Dmitry Peskov said.

One man near the site of Nemtsov's memorial said he thinks the killer was a Putin opponent.

"I don't think Putin did this," he told CNN. "I think this was some radical group that wants to make Putin look bad."

Nemtsov had said he was at risk for assassination. He told the Russian newspaper Sobesednik last month that he was "a little bit" afraid his mother's fears that Putin would have him killed would come true.

But he added, "I'm not afraid of him that much. If I was afraid I wouldn't be heading an opposition party and do what I'm doing."

The consequences of criticism

Critics of Putin have in the past suffered miserable fates.

Last year, a Moscow court sentenced five men to prison for the 2006 killing of Russian journalist and fierce Kremlin critic Anna Politkovskaya.

JUST WATCHED Outspoken Putin critic shot and killed Replay More Videos ... MUST WATCH Outspoken Putin critic shot and killed 02:35

Business magnate Mikhail Khodorkovsky accused Putin of corruption and spent 10 years in prison and labor camps.

And before his death, Nemtsov had been arrested several times for speaking against Putin's government.

Kasparov, chairman of the Human Rights Foundation's International Council, suggested the killing was linked to the Kremlin's own insecurity.

"If you are popular your critics don't have to be shot down in front of the Kremlin," Kasparov tweeted . "Putin is just food in a prison, eat it or starve."