Their identities are secret. They work after sundown, preferring deserted areas of the city. No one knows where they will strike next. They target different neighborhoods each time.

Their mission is simple: to kill Islamic State (IS) militants.

Their targets never vary, but their methods do. Sometimes they use snipers to take out a militant. Sometimes they plant roadside bombs and blow up cars. Sometimes they stab their victims, sometimes strangle them.

They are Mosul's vigilante brigades, shadowy groups of civilians-turned-armed-assassins who risk their own lives to kill IS gunmen -- as well as those who support them.

IS has done its best to eliminate these assassins, tracking down and killing as many of them as it can. But local people in Mosul say these anonymous resistance fighters have had an impact, that IS has covered up the killings and changed how its gunmen operate in Mosul.

Hiding Behind Beards

"Do you know why IS ordered all men [in Mosul] to grow their beards?" the young man asks with a laugh. "It's because they don't want to be recognized."

The young man says he is part of an anti-IS group called the Brigades of Mosul that assassinates IS militants. He tells RFE/RL's correspondent in Mosul that his group has taken out IS gunmen using sniper rifles. Since then, the militants have tried to disguise themselves so they blend in with the public, the young man claims.

The young man, who refuses to give his name, says he and his friends have also planted bombs in Mosul to target IS vehicles. Because of the attacks, IS militants now drive unmarked cars so they are not so visible, he claims.

Resistance Is Not Futile?

Armed anti-IS groups are not a new phenomenon in Mosul, according to RFE/RL's correspondent in the Iraqi city.

They sprang up almost as soon as the militants overran the city last summer. Their names -- the Brigades of Mosul, the Revenge of Nineveh, the Lions of Nineveh, the Brigades for the Liberation of Mosul -- are testament to their members' pride and intense desire to retaliate against IS.

Some of the groups are no longer operating. In some cases, IS tracked down and killed their fighters. Some say they had to disband when they got no support from the Iraqi government in Baghdad.

But neither IS nor a lack of resources has been able to stamp out resistance in Mosul, according to testimony from local residents, who say that armed vigilante groups are causing problems for the militants.

Covering Up

IS tries to cover up the assassinations of its gunmen, witnesses in Mosul say, a sign that the militant group is embarrassed by the killings.

A worker in one of Mosul's morgues tells RFE/RL on condition of anonymity that he receives IS corpses from time to time, though he will not give a precise number.

"IS don't talk a lot about these dead bodies," the morgue worker says. "But if the group has to give some kind of explanation, they say the gunmen were killed in combat or in coalition air strikes."

But the corpses do not show injuries consistent with IS's explanations of their deaths.

They were shot with Kalashnikovs or guns with silencers, or they were strangled or stabbed to death, the morgue worker says.

IS tries to keep the deaths a secret, he adds. The militants only operate under cover of darkness. They bring the bodies to the mortuary and return them to their families only at night.

Hitting Back At Shadows

Mosul residents tell RFE/RL that IS is trying to crush the armed resistance. As usual, the main weapons IS uses to try to force compliance are fear and brutality.

According to the Mosul morgue worker, when an IS militant is assassinated, IS gunmen arrest former security personnel and execute them in public to terrify others. The victims are accused of spying and cooperating with the Iraqi forces.

"[IS] goes crazy when one of theirs is killed," the morgue worker says.

Killing Collaborators

Groups like the Brigades of Mosul do not only target IS militants. They say they also kill those who collaborate with and support them.

The young man from the Brigades of Mosul vows to take revenge on Mosul residents who support IS, including those he says stole and destroyed property in private homes and public buildings.

He and his friends have already killed five local residents, the young man claims. "We did it because they deserve it. They are supporting IS," he says. "Those people betrayed their own city."

'We Are Ready To Fight IS'

Civilians in Mosul say they support the armed groups that kill IS militants.

"Every so often we hear about the killing of one or two IS guys," says Ahmad Ghanim, a Mosul resident whose name has been changed for security reasons. "Most of the people of Nineveh encourage and support such operations. I'm one of them."

It's not true that people in Mosul support IS, Ghanim adds. "Me, most of the men, even women and children are ready to fight IS and support the army or any Iraqi forces or joint forces who came to liberate Mosul," he tells RFE/RL. "The city is suffering because of IS."

A female resident of Mosul, who identifies herself only as M.M., tells RFE/RL that she is ready to take up arms against IS herself.

Failing that, M.M. says she is prepared to help resistance efforts by caring for injured resistance fighters or smuggling weapons on her person.

"It's time to get out and fight these extremists," says M.M., who says she doesn't understand the "global silence" about what is happening in Mosul.

Note: All of the people who spoke to RFE/RL are living in Mosul.