An Arkansas police department is facing a series of federal lawsuits alleging excessive use of deadly force by its officers, in cases that raise serious questions about its public accountability.

Families of victims shot and killed by officers of the Little Rock police department (LRPD) have told the Guardian that there is no effective oversight, and that internal investigations of the killings either covered up or disregarded serious violations of the department's own rules on use of force.

In December, the Guardian revealed how fresh testimony in the fatal shooting of Eugene Ellison, an unarmed 67-year-old African American, by an off-duty Little Rock police officer, cast doubt on the official version of events and the country prosecutor's decision to rule the killing justified.

Ellison's death in December 2010 is now the subject of a federal criminal civil rights investigation, which will also look at accusations that LRPD has a "pattern of misconduct" in its use of force.

Now the Guardian can reveal that two more fatal shootings involving Little Rock officers are now the subject of federal lawsuits. One involves the death of a 28-year-old mentally ill man, Landris Hawkins, pictured, who was shot dead after he threatened to kill himself.

Landris Hawkins, Little Rock Photograph: Michel Laux

Speaking for the first time about the death, his grandmother Neomia Hawkins said that she called 911 expecting help. Instead, she said, police arrived and, after telling him to put down the knife he was holding, killed him.

Hawkins, 63, said: "They could have done something different. They could have tried to calm him. All they did was holler a couple of times and it was all over."

The Hawkins family lawsuit claims the officers failed to follow LRPD rules governing use of force and dealing with mentally ill men and women. It also alleges the force, under police chief Stuart Thomas, has a widespread practice of tolerating violations of rules, allowing acts of excessive force and covering up or disregarding allegations of excessive force.

A request to the LRPD from the Guardian for an interview with Thomas for comment was declined. "As we are in active litigation, we must respectfully decline the interview request," said a spokesman.

Neomia Hawkins, a retired clerical assistant with the Arkansas state police, said she filed the lawsuit because she felt that the department needed to be brought to account. "Something does need to be done. They keep doing it and they get away with it."

Another case involves the shooting of a young father, Collin Spradling, pictured, in July 2008. Police initially told his family he had been shot and killed because he had pointed a gun directly at officers who had a valid warrant for his arrest.

Colin Spradling, Little Rock. Photograph: Michael Spradling

But his father, Michael Spradling, described how two eyewitnesses said they did not see his son with a gun at the time of the shooting. A third witness, according to a video recording shot from one of the officer's cars and seen by the Guardian, said that both of Spradling's arms were pinned behind his back by officers when he was shot. His lawsuit also alleges that the claim that police had a valid arrest warrant was false.

"To this day I have no faith in the police I see the news and I hear about a shooting and I'm absolutely suspicious of it," said Michael Spradling.

Little Rock, with a population 195,314, is one of the most violent cities in the US. It has a violent crime rate of 1,490 crimes per 100,000 people, compared to a US average of 386.3 per 100,000, according to the FBI. The city reported 440 robberies per 100,000 people in 2011, compared to the national average of 113.7.

However, the LRPD also has rules governing potentially dangerous situations such as dealing with mentally ill people, when to use deadly force and indeed, when to use force in general. Critics say that the system is broken and urgently needs fixing, in particular the internal investigations of police killings.

"We think instances of police misconduct are likely to continue as long as matters such as these are not subject to an appropriate review process," said lawyer Michael Laux, who, along with his partner David Eisbrouch, of Balkin and Eisbrouch LLC, is representing all three families.

"Word gets around. And when officers become aware that instances of excessive force are not reviewed appropriately, it would seem to create a very dangerous precedent for the city."

In the last five years, there have been 32 shootings involving LRPD officers, according to files released in response to a freedom of information request by Laux.

It is difficult, however, to make any comparison with similarly sized cities as no central statistics are kept on fatal shootings by police, except in cases involving the shooting of a felon. The FBI, which records other crimes across the country, does not record police shootings, other than "justifiable homicides" – acts defined as "the killing of a felon by a law enforcement officer in the line of duty".

In a case which critics say had a chilling effect on relations between the city's black community and police, Joshua Hastings, a former LRPD officer, was charged with manslaughter last year after killing Bobby Moore, a black 15-year-old whom he suspected of breaking into cars.

While a prosecutor ruled that, in the Hastings case, use of force was not justified, and he was suspended from the force, it later emerged that in his six years as an officer he had been suspended five times. A report by KARK4 News found that, a supervisor said Hastings had "continuously show poor decision-making and unprofessionalism as a Little Rock police officer" and that he had "great concern for his mental stability and decision making as a Little Rock police officer".

Hastings is the son of Lieutenant Terry Hastings, a veteran of the force who is also accused in the lawsuit brought by the family of Michael Spradling of falsely telling them that officers had had a valid warrant for his son's arrest.

Max Brantley, former editor of the Little Rock-based Arkansas Times, who now writes a blog for the liberal weekly paper, said that a belief that the LRPD misuse their authority has "been a leitmotif running through my 40 years here."

"My own limited experience of cops is that they are very professional," said Brantley, who has a pile of files on his desk relating to misuse of force and other misconduct allegations at LRPD. "But the cases mount. There's a swagger in how they operate. The police attitude is 'we own this space'. They are prone to exercise authority pre-emptively."

Brantley told the Guardian the LRPD has also been accused of excessive use of force in a number of non-fatal cases, including one involving Lieutenant David Hudson, which is the subject of another civil rights lawsuit against the department.

Hudson was suspended without pay for a month in November last year after beating up a customer outside a restaurant, while employed as a private security guard. The customer, Chris Erwin, was charged with resisting arrest, drunken conduct and public trespass, but the charges were later dropped. The incident was caught on video.

In a blogpost on the case, Brantley wrote that the suspension sent a "chilling signal" to the public. "Even with multiple witnesses and video, a beating of a non-criminal for backtalking an officer acting as a private security guard is not a firing offense. It gives the public every reason to wonder what happens when the lights are out, no videos are running and confrontations occur in poorer parts of town."

In another case in April 2012, Joe Thompson, the Arkansas surgeon general, was arrested after a confrontation at his home. Charges against him, including disorderly conduct and resisting arrest, were later dropped after audio files of the incident, which appeared to counter the police version of events, circulated on the internet.

Brantley said the time has come for a "thorough review of Little Rock police department's use of force, lawsuits or no lawsuits." He said: "I don't think you can have credibility in a police investigation in this town any more. You need to look at the use of force and individual officers."

Tim Lynch, of the Cato Institute's police misconduct reporting project, said that when questions have been raised over a police shooting, by victims or witnesses, it was the responsibility of the police chief or prosecution to turn it over to an outside agency for review. "This is what we have found to be best practice," Lynch said.

Lynch said that he had been calling for more transparency of the disciplinary records of police officers, because such records can identify "red flags" if there are problems with individual officers. He said that the disciplinary records are "shrouded in secrecy" and often, lawsuits against the police can also be settled in secret, leaving outside observers unable to judge whether a pattern of bad behaviour exists and how a force is reacting.