Was Sandra Bland murdered?

That’s what distraught family and friends of the 28-year-old woman are asking after she was discovered hanging by a plastic garbage bag in a Texas jail cell three days after she was arrested during a routine traffic stop that turned confrontational.

The coroner’s office quickly ruled Ms. Bland’s July 13 death a suicide. However, in a stunning development late Wednesday, the Texas district attorney’s office said the autopsy was defective and a second one is needed.

Ms. Bland’s family said she moved to Texas from Chicago and was excited about starting a new job at her alma mater, Prairie View A&M University, and would not have taken her own life. However, they acknowledged that she had posted a video blog several months ago saying she was struggling with depression.

Her family and other advocates have called for U.S. Attorney General Loretta Lynch and the U.S. Justice Department to launch a federal probe.

The Texas Rangers and the FBI are jointly investigating her death. An independent autopsy has been ordered.

The case has gained national attention, with questions swirling about possible tampering with the dashcam video retrieved from the arresting Texas trooper’s car and the alleged racist history of the sheriff in charge of the Waller County Jail.

At a news conference Monday, Waller County District Attorney Elton Mathis said there were “too many questions” to determine how she died and that “this is being treated like a murder investigation.”

On Monday, authorities released a three-hour video taken from outside Ms. Bland’s jail cell. The video shows a period of about 90 minutes with no activity in the hallway leading to her cell. The video does not show the inside of her cell or even her cell door.

It then shows a deputy reacting to what she sees while looking in the cell, triggering a frenzy of activity.

Contradicting arguments that she had killed herself, authorities found a Bible and a book titled “101 Ways to Find God’s Purpose for Your Life” beside her body.

Protesters have staged demonstrations outside the jail, with her case now taken up by people who say it is the latest example of racial bias and excessive force by law enforcement against African-Americans.

A memorial for Ms. Bland held at Prairie View A&M on Tuesday night drew about 300 people.

“I am angry because of the way she died,” her mother, Geneva Reed-Veal, said at the memorial. “All I want to know is why.”

Ms. Bland was arrested July 10 by state Trooper Brian Encinia after he pulled her over for allegedly failing to signal during a lane change.

A 52-minute dashcam video of the incident, released by the Texas Department of Public Safety on Tuesday, starts with the end of an unrelated traffic stop.

The trooper then makes a U-turn and ends up behind Ms. Bland’s Hyundai Azera with Illinois plates. Her car can be seen in the left-hand lane of two lanes as Trooper Encinia’s patrol car approaches her. She then moves from the left lane into the right lane, and the trooper pulls her over about 4:30 p.m.

During the stop, the trooper asks Ms. Bland if she is irritated. She replies that she is upset about being stopped for such a minor issue.

The trooper asks her to put out her cigarette. She refuses, saying she is in her own vehicle, and he tells her she is under arrest.

At that point, the video shows the trooper demanding she get out of her car. He tries to pull her out. He then points a Taser and shouts: “I will light you up.”

When she finally steps out of the vehicle, the trooper orders her to the side of the road. There, the confrontation continues off-camera, but it is still audible. The two keep yelling at each other as the officer tries to put Ms. Bland in handcuffs and waits for other troopers to arrive.

Still off camera, Ms. Bland can be heard protesting her arrest, repeatedly using expletives and calling the trooper a “pussy.” She screams that he’s about to break her wrists and complains that he knocked her head into the ground.

Ms. Bland was taken into custody and charged with assaulting an officer, a felony.

Three days later, authorities found her hanging in her jail cell, sparking national outrage and many unanswered questions about her death.

Trooper Encinia, 30, has been put on administrative leave after a preliminary investigation found he violated the Texas Department of Public Safety’s traffic stop and courtesy procedures during the stop.

He has been a state trooper with the department for a little more than a year. He previously worked as a district chief at a fire department, according to his LinkedIn profile.

Critics also are accusing the Texas Department of Public Safety of editing the dashcam tape because there are several spots in which cars and people disappear and reappear.

Also, the audio ends more than a minute before the video images do.

In a statement to reporters Wednesday morning, Tom Vinger, press secretary for the state police agency, said the video was unedited and blamed the problem on technology. Officials promised to upload a glitch-free video of the traffic stop later on Wednesday.

Waller County Sheriff Glenn Smith, who made the initial public comments about Ms. Bland’s death, reportedly was suspended and later fired as police chief of Hempstead, Texas, in 2007 because of complaints of racism.

Ms. Bland’s body has been flown back to Chicago by her family, with her funeral scheduled for 11 a.m. Saturday, July 25, at the DuPage African Methodist Episcopal Church in Lisle, Ill., where she was active in several of the church’s ministries, including the Girl Scouts, Sisters in Praise & Voices of Tomorrow and with the Women’s Day Committee.

The Rev. James F. Miller, pastor of the suburban Chicago church, posted a note on the church website stating that in tribute to Ms. Bland, the church had a “Prayer Walk” Sunday around the grounds, “as a public expression of unity and a desire for justice in her case along with so many others.”

He also noted that a petition urging “a comprehensive investigation with integrity” regarding the circumstances surrounding her death would be circulated.