after weeks of protests in Hong Kong.

A group of men in white shirts attacked demonstrators

heading home after an antigovernment march.

Protesters said that officers ignored their repeated calls

for help.

Under public pressure, the police defended their actions

and acknowledged small gaps in their response.

But their conduct left many wondering

why they didn't do more.

We reviewed hours of video and spoke to a dozen witnesses

to look at how officers failed to stop the violence.

Our investigation reconstructs what happened that night.

It's Sunday, July 21.

Antigovernment demonstrations are happening on

Hong Kong Island.

Protests like this one have been going on for weeks.

They started in opposition to a controversial extradition bill.

But now, protester demands are growing.

Meanwhile, 18 miles away in Yuen Long —

a town near the border with mainland China —

a crowd has gathered.

Most are wearing white T-shirts

and rallying in support of the government.

A prominent, pro-Beijing lawmaker is there.

Traditional villages in this region have a long history with

Hong Kong's organized crime societies known as triads.

Experts say that while the triads have

a patriotic reputation, they're essentially thugs for hire

and will work for whomever pays them the most.

Members of triads have been accused of violence

against antigovernment protesters in the past

and some of the people behind the night's violence

were later identified by police as triad members.

Now, let's go back to what happened that night.

It's just past 10 p.m. where the pro-government rally is

taking place and things are getting violent.

Videos show at least three individuals being beaten

by men in white T-shirts.

Lam Cheuk-ting, a lawmaker who supported some

antigovernment protests, says

he learns about the violence and calls the police at 10:22 p.m.

Outside, at around 10:40 p.m.,

we see men in white walking towards the Yuen Long

train station, where antigovernment protesters

from Hong Kong Island have been arriving on their way home.

Lam Cheuk-ting warns them of the danger outside.

But the mob comes inside the station.

And they begin to taunt the protesters.

Moments later, we see two officers leaving the scene.

Police said that they had received the first call

from the train station at 10:41 p.m.,

and that these officers left to call for backup.

But once they leave, the violence escalates.

[screaming]

A reporter is

livestreaming when she's attacked by the mob

People run up the stairs trying to escape.

The men in white chase them onto a train.

Some men hold open the train doors

while others beat people inside.

Bystanders are caught up in the melee.

Meanwhile, others are trying to get help

at two nearby police stations.

But when some arrive at Yuen Long police station,

officers are seen closing the gates.

And at another police station,

officers sit inside as people bang on the door.

Police later said that officers closed the doors

for security reasons.

Back at the train station,

the violence continues.

Finally, the train leaves.

Lam Cheuk-ting is among the injured.

The violence lasts for about 20 minutes.

In our analysis, we never see any police officers

at the scene during this period.

It's now 11:15 p.m.

As some of the men in white T-shirts run away,

police do not appear to pursue any of the perpetrators.

Protesters confront police for their lack of action

for nearly half an hour.

At this point, the officers leave the station,

entrance gates close behind them.

But at 12:26 a.m., the mob in white T-shirts is back.

They break through the train station gate

and start to beat people again.

Eight minutes later this dash cam video

shows men in white T-shirts casually walking past

police vehicles while officers walk away.

White-shirted men continue to beat protesters outside.

Some of the men gather at a parking lot.

In this video,

we see police entering the train station as the men

in white are just outside.

Police surround the area for the next couple hours.

It's 2:35 a.m., we see two men still holding their weapons,

chatting with officers.

This is important because hours later, police officials

said that they did not see anyone holding weapons.

Finally, around 4 a.m. several of the men in white T-shirts

drive away.

No one was arrested overnight.

At least 45 people

were injured during the attack at the Yuen Long station.

Police have admitted that they were informed

about a potential attack in Yuen Long in advance.

The Hong Kong police commissioner

said the antigovernment protests were the reason

for the slow police response.

The police have since arrested 12 suspects for the attacks,

many of them with triad connections.

Still, protesters say officers could have done more

to prevent the violence.

Almost a week after this incident,

thousands gathered in Yuen Long to protest the attack and the

police response.

This time, the men in white shirts

appeared to remain on the sidelines.

And it was the police who countered protesters

with force.