AMNA NAWAZ: The March 22 terrorist attack# on Crocus City Hall killed 145 people and## injured hundreds, the deadliest attack in Russia# in 20 years.

Russia blamed the U.S. and Ukraine,## despite specific U.S. intelligence# shared with Russia of an impending## attack by ISIS Khorasan and that group having# claimed responsibility after the attack.

Nick Schifrin is back now with a look at# what a U.S. warning and Russia's subsequent## unfounded accusations say about the U.S.-Russia# relationship and Russia's intelligence services.

NICK SCHIFRIN: Before the# band could take the stage,## what was supposed to be a raucous# concert became a living nightmare.

(GUNSHOTS) NICK SCHIFRIN: Those they didn't# slaughter, they tried to burn alive.

(GUNSHOTS) EFIM FIDRYA, Concert Attack Survivor (through# translator): These screams of people,## I understood, shots and screams# from people who are frightened,## who are now maybe dying.

This# is pretty hard to ever forget.

NICK SCHIFRIN: Efim Fidrya bought# tickets for the rock group Picnic## to celebrate his wife Olga's birthday.# The shooting started as they arrived.

EFIM FIDRYA (through translator): We headed# toward the entrance to the hall and almost## immediately heard shots.

We ran down the stairs# and took refuge in the toilet stall.

Someone was## praying.

Someone was calling their loved ones,# screaming emotionally to get us out of there.

After a while, smoke began to seep in.# We decided to leave.

A girl's body lay## near the escalator that led to# the first floor.

She was dead.

NICK SCHIFRIN: You mentioned how# it helped when you heard that the## terrorists were arrested.

President Putin# has described them as radical Islamists,## but having a connection to Ukraine.

What# do you think when you hear that statement?

EFIM FIDRYA (through translator): Considering# that their behavior is different from typical## radical Islamists -- they did not try to commit# suicide.

They did not take hostages and went## towards the Ukrainian border.

It is possible that# the official version will actually be correct.

NICK SCHIFRIN: Putin provided# the initial official version.

VLADIMIR PUTIN, Russian President# (through translator): The crime was## committed by radical Isl.. is trying to convince its satellites# there is supposedly no trace of Kyiv.

NICK SCHIFRIN: Today, Russian investigators# revealed what they said was a suspect's phone,## with photos inside of iconic symbols of# Ukrainian resistance and Crocus City Hall.

And Russia's top intelligence officer,## Alexander Bortnikov, has blamed the attack# on Ukrainian and American intelligence.

ALEXANDER BORTNIKOV, Director, Russian Federal# Security Service (through translator): We believe## that the action was prepar.. Western special services.

The Ukrainian special# services themselves are directly related to this.

NICK SCHIFRIN: The U.S. and Ukraine# both say only ISIS was responsible.

Two U.S. officials tell "PBS NewsHour"# the U.S. warned Russia of a possible## terrorist attack and named specific targets,# including Crocus City Hall.

On March the 7th,## the U.S. Embassy in Moscow made# the warning public.

"Extremists## have imminent plans to target large# gatherings in Moscow to include concerts,## and U.S. citizens should be advised to avoid# large gatherings over the next 48 hours."

Despite the U.S. specificity, Russia would# later call the intelligence -- quote -- "too## general."

And before the attack,# Putin disparaged the U.S. warnings.

VLADIMIR PUTIN (through translator): All# these actions represent outright blackmail## and the intention to intimidate# and destabilize our society.

ANDREI SOLDATOV, Russian Investigative# Journalist: There is a sense and a climate## of conspiratorial thinking, which is quite# widespread in Russian society these days,## that Russia is a besieged fortress.

NICK SCHIFRIN: Andrei Soldatov is a senior# fellow at the Center for European Policy## Analysis and an investigative journalist# who covers Russia's security services.

ANDREI SOLDATOV: They have this sense# of extreme fragility of the Russian## state.

And they think that -- again, that# this state, which is extremely fragile,## is under constant attack.

So, even if they# see and they have this evidence that the## real radical Islamists are behind that# attack and they caught them red-handed,## they would try to develop a bigger theory that# there are some hostile forces behind this facade.

NICK SCHIFRIN: Soldatov points out that Russia# did make some arrests immediately after the## U.S. warning.

But he says Putin has created# a climate where it's impossible for society## to question the security services and impossible# for intelligence officers to work with the West.

ANDREI SOLDATOV: People who became too# close, from the Russian point of view,## to the Americans and shared# too much with the Americans,## in terms of counterterrorism, these people# were immediately punished and sent to jail.

The other problem is that, if# you shield your security and## intelligence services from any kind of criticism,## your security services will always fail you,# because they feel completely impenetrable.

NICK SCHIFRIN: And so Putin's besieged# fortress failed to protect its own people,## the price, 145 innocent lives.

For the "PBS NewsHour," I'm Nick Schifrin.