The deaths in recent days of two Black men in Tennessee, one in the custody of police and the other at the hands of national guard troops, has prompted the National Association for the Advancement of Colored People to demand the suspension of the so-called Memphis Safe Task Force, Donald Trump’s anti-crime alliance of federal, state and local law enforcement.

In a letter to acting attorney general Todd Blanche on Friday, the NAACP also demanded “a thorough, transparent federal investigation” into the actions of the taskforce, which it said has surged federal and military officers into Memphis with insufficient training for civilian policing.

The action follows the death by shooting of Tyrin Johnson, a 20-year-old new father, by two Tennessee national guard troops in an early morning incident on Sunday; and the discovery of the body of Darius Chappell, a 34-year-old father of three young sons, in a cell at the Montgomery county jail two days earlier.

Chappell was taken into custody on 29 June after what police said was a “use of force” incident in Clarksville. Video of his violent arrest circulating on social media showed a police canine appearing to bite him while he was being held on the ground by several officers.

Authorities have revealed few details about either incident. In Johnson’s case, the Tennessee bureau of investigation (TBI) asserted he had been in possession of a handgun and had fired shots.

Meanwhile, the Clarksville police department said it had launched an investigation into the deployment of a K9 during Chappell’s arrest, and had placed an officer on administrative leave.

The taskforce, established in an executive order by Trump last year as part of the president’s push to reduce perceived soaring crime rates in Democratic-run cities, has been linked to at least four deaths this year.

In the letter, the NAACP said it was aware the TBI was investigating Johnson’s death, but that it was “not enough”, particularly in the light of “conflicting reports” issued by various agencies about what happened.

“The federal government has unique expertise and skill in investigating law enforcement misconduct, and in the case of Mr Johnson, there are challenges that arise given the involvement of federal officials,” it said.

Regarding Chappell’s death, the letter noted “no officers have been charged, and an official cause of death has not been issued”.

It said: “There is a need for transparency into the fact surrounding Mr Chappell’s death. The public deserves facts about the force that was used and the extent to which Mr Chappell was offered medical care while in custody.”

Addressing Blanche directly, the letter said: “You swore an oath to support, protect, and defend the Constitution of the US. Despite this, during your tenure, we have seen the justice department fully retreat from investigating these matters.

“The department has abdicated its responsibility to prosecute officials who violate people’s civil and constitutional rights.”

Derrick Johnson, president and chief executive of the NAACP, said in a statement accompanying the letter that the group was seeking “transparency to affected communities and full accountability and justice”.

He said: “The Department of Justice cannot continue to stand by while Black lives are taken. The DoJ has a fundamental responsibility to support, protect, and defend the US Constitution, and that Constitution includes the rights, the safety, and the futures of Black Americans.

“When officers fail to deliver equal protection under the law, the federal government must step in with its investigative authority. We will not allow the names of Darius Chappell and Tyrin Johnson to be overlooked.”

The NAACP, he said, conducted a survey of Memphis residents and found 63% of respondents strongly disapproved of the deployment of the national guard, and 54% thought it had significantly undermined the safety and trust of residents.