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Sandra Bland was found dead in her cell, three days after she was arrested when a traffic stop for a minor violation escalated into a confrontation with a white trooper.

Police said the 28-year-old black woman was found hanged with a plastic rubbish bag around her neck.

A post mortem showed her death was consistent with suicide but officials have said they are handling the investigation like a murder probe.

Activists claim her death is the latest example of racial bias and excessive force by US law enforcement.

Now conspiracy theories have emerged on social media, suggesting the Chicago woman may have been dead when her mugshot was taken.

Other have raised suspicions after apparent inconsistencies were spotted in her arrest documents.

Bland was arrested over a minor traffic violation in Prairie View, Texas, on July 10.

Police dashboard camera footage shows the incident escalating from a warning for failure to signal a lane change to her being placed in handcuffs and a physical altercation with the white trooper.

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As Bland's case gained traction on Twitter, users questioned perceived abnormalities in her mugshot, even suggesting that she may already have been dead when the picture was taken.

Some said the shadow on the surface behind Bland's head and the positioning of her shoulders showed she may have been lying on the floor, rather than sitting up straight.

Others questioned why her mugshot was not taken in the clothes she was arrested in.

Only one mugshot, showing Bland facing forwards, was initially released by Waller County Jail.

When it was questioned why a second, side-on shot was not taken, as is usually the case, police released a second but it appears to be too dark to be decipherable.

Other users have highlighted apparent inconsistencies in Bland's arrest booking documents.

Jailers on duty when Bland was admitted felt she was not a suicide risk based on their observations and her statement on the questionnaire that she was not depressed at the time.

But images emerging on Twitter appeared to suggest contradicting information between the suicide risk assessment and the inmate intake sheet.

Preliminary tests found high levels of marijuana in Bland’s system, although officials are seeking additional tests to confirm when and how much she might have smoked or swallowed.

"The evidence that we reviewed up to this point supports those findings," Waller County Assistant District Attorney Warren Diepraam said of the initially suicide ruling.

Diepraam said it was unclear whether Bland ingested marijuana before or after she was jailed.

Waller County Sheriff Glenn Smith, whose office operates the jail where Bland died, has searched her cell for evidence of marijuana and found nothing, Diepraam said.

However, he added that the high level of the drug in Bland’s system could have influenced her mood while in jail.

Listen to Sandra Bland's haunting voicemail from jail days before she was found hanged

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"It is a mood amplifier, so it is relevant in our opinion to determine whether or not marijuana may have played a role in her death as well, exacerbating existing conditions," he said.

Bland had lacerations and abrasions on her wrists that could have been received when she was handcuffed during her arrest, he said. She also had about 30 healing cuts on her left arm that occurred two to four weeks before, he said.

An attorney for the Bland family, Cannon Lambert, before Diepraam’s press conference called the autopsy "defective."

"The family’s confidence is shaken by the continued discrepancies that are surfacing," Lambert said in an email.

The family has maintained Bland was excited to start her new job at her alma mater, Prairie View A&M University, and was not suicidal.

On Tuesday, the Texas Department of Public Safety released the 52-minute dashboard camera video of the traffic stop on its YouTube channel.

(Image: Reuters)

(Image: Reuters)

When the trooper, identified as Brian Encinia, asked Bland to put our her cigarette, she refused and the officer asked her to get out of the car.

In the rest of the video, Encinia pulled out his Taser, Bland got out of the car, and then they are out of view and Bland can be heard protesting her arrest.

Encinia as been put on desk duty for allegedly violating protocol in the arrest.

Jailhouse video released on Monday showed a space of about 90 minutes between the last physical check of Bland in her cell, and when she was discovered dead at about 9am.

In a separate case, in Alabama, protesters rallied outside the jail in Homewood, a suburb of Birmingham, questioning the death of an 18-year-old woman that police say committed suicide in jail last week.

Kindra Chapman, who had been arrested on robbery charges, died in custody on July 14, according to WBRC-TV and the news website AL.com.

Authorities said she was found in her cell with a bed sheet tied around her neck in a suicide attempt.