He spends most of his days in "nearly total isolation," according to his attorneys, locked behind a heavy steel door in a tiny cell in the most restricted wing at Fort Devens medical prison 40 miles outside Boston.

His only visitors have been members of his legal team and his two older sisters — though the sisters have come to see him only a handful of times and always under the observation of an FBI agent. He has not been allowed to mingle with or talk to any other inmates — either verbally or through notes. His only other regular contact has been with prison personnel, who slide meals through a slot within a thick glass observation window in a corner of his cell door.

The closest Dzhokhar Tsarnaev has come to experiencing the world beyond his cell in more than 500 days has been through "very limited access to a small outdoor enclosure," according to court records. And that's only "on weekdays, weather permitting." But that will soon change.

Tsarnaev, the surviving suspect in the April 2013 Boston Marathon bombings, is scheduled to appear in a federal courtroom in Boston this week. His attorneys have informed prosecutors with the U.S. attorney's office in Boston that he'll attend a Dec. 18 status hearing in the bombing case — the last before his highly anticipated trial, which is set to begin Jan. 5.

The hearing will mark the first time Tsarnaev has been seen in public since July 2013, when he was formally charged with helping to plot and carry out two bombings near the marathon's finish line. Three people were killed and several hundred more injured in the attack, which Tsarnaev is accused of having committed with his older brother, Tamerlan, who was killed in a confrontation with police four days after the bombings. The brothers are also accused of shooting and killing a Massachusetts Institute of Technology police officer while on the run from police. Tsarnaev, who has pleaded not guilty in the case, faces the death penalty if convicted.

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CLICK IMAGE for Boston Marathon bombings slideshow: (Dan Lampariello/REUTERS)

Tsarnaev's appearance will undoubtedly renew public interest in the bombings, which are still shrouded in mystery. Nearly two years after the attacks, the public has gained little insight into how and why the Tsarnaev brothers allegedly came to commit such a terrible crime — or whether others knew or were involved. An FBI agent involved in the investigation testified under oath during an October trial of a Tsarnaev associate that investigators still did not know where the two bombs had been built.

Another major unknown is exactly how Tsarnaev's attorneys plan to defend their client in the face of what appears to be overwhelming evidence linking him to the attacks. Prosecutors are expected to present previously unseen surveillance video of Tsarnaev and his brother near the marathon's finish line. Footage reportedly shows Tsarnaev dropping a backpack on the ground near Martin Richard, an 8-year-old Boston boy who was killed by the second blast, moments before the bomb went off.

Tsarnaev's attorneys have resisted revealing too much of their strategy — even fighting to delay their list of witnesses until the last possible moment. (The latest deadline is Dec. 29 — a week before jury selection is scheduled to begin.) But court records have suggested that Dzhokhar's legal team plans to paint Tamerlan Tsarnaev as the mastermind of the attacks and cast their own client as someone from an intensely troubled family who was unduly influenced by his older brother.

Yet it's unclear if Tsarnaev is participating in or approves of that line of argument — or how much help his attorneys are receiving from him or his family for his defense. In August, Ailina Tsarnaeva, one of the suspect's older sisters, who is facing legal problems of her own, told a Boston reporter, "My brothers were framed. Everybody knows that."

The public may get a glimpse of how the 21-year-old suspect views his defense at his court appearance Thursday. While Tsarnaev is not required to attend the Dec. 18 hearing, prosecutors with the U.S. attorney's office in Boston, who are handling the case, specifically requested that he attend. They argued it would be his last opportunity to state whether he has any problems with his case or attorneys before the trial formally begins — and they say his concerns should be raised now, as opposed to on appeal.

This courtroom sketch depicts Boston Marathon bombing suspect Dzhokhar Tsarnaev standing with his lawyer Miriam Conrad, left, before Magistrate Judge Marianne Bowler, right, during his arraignment in federal court Wednesday, July 10, 2013 in Boston. (Jane Flavell Collins/AP Photo)

One of the enduring mysteries in the case is Tsarnaev himself. Many of his friends and former teachers have struggled to reconcile the popular teenager they knew with someone who allegedly committed such a terrible crime. And little has been revealed about Tsarnaev in the 20 months since he was captured, hiding and wounded by gunfire, in the back of a boat in the Boston suburbs four days after the bombings.

Shortly after his arrest, a federal judge imposed a gag order, restricting what could be released about the evidence in the case and about the suspect. But some details about Tsarnaev's life behind bars have emerged, mostly in the hundreds of pages of court filings exchanged between federal prosecutors and his attorneys in advance of the January trial.

In court 17 months ago, Tsarnaev, then just 19, appeared fidgety and spoke with a distinct foreign accent that some of his closest friends from high school, who attended the hearing, said he had never used before. His left arm was bandaged, and the left side of his face appeared swollen and slightly frozen — evidence of the injuries he'd apparently sustained when being taken into custody by police roughly three months earlier. Even from the back of the room, reporters could see a large scar on his throat. Subsequent court filings have detailed how badly injured Tsarnaev was. In a filing from last May, his attorneys wrote that he arrived at Boston's Beth Israel Deaconess Medical Center in "critical condition ... with gunshot wounds to his head, face, throat, jaw, left hand and both legs." Within minutes of arrival, Tsarnaev's "mental status suddenly declined," and doctors had to perform an emergency tracheotomy to keep him alive, according to the filing. One of the shots had fractured the base of Tsarnaev's skull, and his attorneys, citing hospital records, said another gunshot "likely caused traumatic brain injury… Damage to the cranial nerves required his left eye be sutured shut; his jaw was wired closed and injures to his left ear left him unable to hear on that side."

Tsarnaev's defense team cited the injuries in an effort to suppress written statements, including an apparent confession, that he made to FBI agents before he was read his Miranda rights. (He was unable to physically speak because of the intubation.) But federal prosecutors had previously said in court records they don't plan to use the statements he made at the hospital during the trial — raising the question of whether his defense team is laying the groundwork to use his health or mental state as a way of potentially saving him from the death penalty.

In this Friday, April 19, 2013 photo provided by the Massachusetts State Police, Boston Marathon bombing suspect Dzhokhar Tsarnaev leans over in a boat at the time of his capture by law enforcement authorities in Watertown, Mass. (Sean Murphy/Massachusetts State Police/AP Photo)

It is unclear how much Tsarnaev has recovered from his injuries or whether he continues to be under medical care. Miriam Conrad, a federal public defender in Boston who is one of the attorneys representing him, declined to comment on the case when contacted by Yahoo News. Mary Long, a public information officer at Fort Devens, also declined to comment on Tsarnaev.

Long and other Bureau of Prisons officials say they are limited in what they can say about Tsarnaev because he is being held under "Special Administrative Measures," known as SAMs. These measures allow the attorney general to enact special restrictions on prisoners whom the government considers to be a significant risk, and in the aftermath of 9/11, they've mainly been applied to terrorism suspects.

Attorney General Eric Holder ordered the restrictions placed on Tsarnaev in late August 2013 — more than four months after his arrest — describing him as a danger to society. The memo, personally initialed by Holder, said Tsarnaev had "reaffirmed his commitment to jihad" during his initial interrogation by the FBI and "expressed hope that his actions would inspire others to engage in violent jihad." It also cited his "widespread notoriety," pointing to the fact he'd received nearly 1,000 pieces of unsolicited mail.

Tsarnaev's attorneys, in an October 2013 filing pushing to lift the restrictions, pointed out that most of the letters were from individuals who had written him urging him to convert to Christianity and that the bombing suspect had not replied to any of the correspondence. And they questioned why the government had waited months to implement the restrictions, even though Tsarnaev had been a model prisoner.

Under the rules, Tsarnaev is allowed to write one letter — three pages, double sided — and to make one phone call per calendar week to his immediate family. His letters are read, and the calls are recorded by federal officials. Under the rules, his family is not allowed to speak about what he tells them. The calls also cannot be recorded by his family.

Tsarnaev is barred from speaking to the media, "in person; by telephone; by furnishing a recorded message; through the mail; his attorney, or a third party; or otherwise," according to Holder's memo. He is allowed to watch television and listen to the radio — but his cell at Fort Devens does not offer either. If he reads newspapers or magazines, they are first stripped of letters to the editor, ads or any other elements that could be used to pass coded messages. He has access to books — but the prison is not allowed to say what he reads.

In this April 25, 2013 photo, Zubeidat Tsarnaeva, right, mother Boston Marathon bombing suspects Tamerlan and Dzhokhar Tsarnaev, speaks at a news conference beside the suspects' father Anzor Tsarnaev, left, in Makhachkala, capital city of the southern Russian province of Dagestan. Tamerlan and Dzhokhar Tsarnaev befriended Donald Larking, a brain-damaged anti-U.S. government conspiracy theorist, through their mother's health care aide job years before the deadly attack, a lawyer for Larking's family said Tuesday, Aug. 6, 2013. (Musa Sadulayev/AP Photo)

Court records show that Justice Department officials have seemed particularly concerned with Tsarnaev's communications with his family. In ordering the restrictions, Holder pointed to a May 2013 phone call Tsarnaev made to his mother in Dagestan, Russia, which she recorded and played for reporters. In the call, Tsarnaev, who was speaking in Russian, told his mother that he was "not in pain" and that supporters had deposited $1,000 into his prison commissary account. The call, Holder wrote, was "an apparent effort to engender sympathy" for the bombing suspect.

The government has refused to allow Tsarnaev to see his sisters without an FBI agent in the room — even when one of the suspect's attorneys is present and it is for his legal defense. In recent months, Tsarnaev's attorneys have pushed for the restrictions to be loosened, arguing it is hampering their ability to mount a defense with the help of his family. But the prosecutors have refused to let up.

In February, prosecutors said an FBI agent overheard Tsarnaev make a "statement to his detriment" during one visit with his sister — though they did not specify exactly what was said. The government mentioned the incident in a court filing arguing against relaxing restrictions on Tsarnaev's ability to meet with his family privately, suggesting it was a security risk. His defense team pushed back on the allegation, saying the government had distorted the remark and that the SAMs prevented them from specifically refuting what their client had said.

Later in the spring, the government again refused to loosen the restrictions, according to Tsarnaev's attorneys, because prosecutors said his mother, during a phone call with her son that was monitored by the FBI, made reference to "unspecified friends of his brother Tamerlan" being present during the call. His attorneys called the complaints "unfounded, generic speculation" and "paper-thin concerns about security" intended to frighten the public about their client.

But the restrictions have remained in place, leaving little more than snippets of court records to indicate how Tsarnaev has spent the last 17 months behind prison walls. This week in a Boston courtroom, the public will be able to judge for itself.