On Thursday, five of the six court rulings and motions filed that day in the Dzhokhar Tsarnaev case — four motions from attorneys and one order from the judge — were filed under seal.

Under seal means there is a record that something has been filed, but the public can't actually see what the filings say.

This has been the norm in the months leading up to the high-profile federal terrorism trial, which is set to start Jan. 5.

As of Friday evening, there have been 777 filings made in the accused Boston Marathon bomber's case dating back to April 21, 2013. In the last month (from Nov. 19 to Dec. 19), there were 122 filings with 74 (or 60 percent) marked as sealed. The filings include motions, responses to motions, documents and orders from the judge.

Most filings were also under seal in the previous month (from Oct. 18 to Nov. 18), where 55 percent of the filings (or 25 of 45) were sealed.

Why have there been so many sealed filings in the Boston Marathon bombing case?

In short, it's because of the judge's rules and the nature of the trial.

U.S. District Court Judge George O'Toole Jr. set the rules that way, according to now-Judge David Frank, who wrote about the issue in August when he was managing editor of Massachusetts Lawyers Weekly.

O'Toole acknowledged concerns about the large number of sealed filings at a September 2013 hearing. He said those concerns stemmed from a lack of familiarity with local rules, Frank wrote. To submit a sealed filing, a lawyer has to first file a sealed motion explaining why they should be allowed to do so, Frank explained. This basically means that the lawyers seeking to file a sealed motion end up filing two sealed motions in the process (that's of course if O'Toole grants the first sealed motion).

On Thursday, during a status hearing for Tsarnaev — who made his first court appearance since 2013 — Judge O'Toole again addressed the issue of sealed filings. O'Toole said he wants to make as many of the filings public once a jury is selected. O'Toole said he didn't want to prejudice the jury before one was seated.

Another reason for the large number of sealed documents is the nature of the case, according to Brad Bailey, a longtime criminal defense attorney and former state and federal prosecutor.

"You have the added both unusual and extra factor that on top of this being a case involving issues of terrorism and national security that it’s also a death case," Bailey said.

He added that there may be information related to those aspects of the case that is too personal or not appropriate for public dissemination at this time — especially with the high number of victims in this case.

"That’s 260 separate people with very personal information and personal issues which also to a degree needs to be protected," Bailey said. "So because of that, because it's a national security terrorism case and a death case with a huge number of victims that’s one of the reasons you’re seeing a high volume of sealed documents in this case."

We asked Bailey to expand on the high number of sealed filings in this case and to provide insights into what this means for the Tsarnaev trial. Here's what he said (lightly edited):

Is the high number of sealed filings unusual?

It’s not unusual certainly in cases involving organized criminal activity and we see potential witness lists held back the last minute or filed under seal in cases like those. And we see other evidence that for the time being may be held back really out of concerns of personal safety of potential witnesses and/or their families.

It’s not unusual in cases of a national security nature as well. We certainly don’t see a lot of those [cases] in this country, but in this very case there were motions that were filed that related to intelligence surveillance techniques and data, and my guess is some of the responses to the motions filed by the defense in that regard are being protected and will remain under seal because there are things and techniques that are in use that the government does not want to be compromised simply in responding to certain discovery requests.

How much of the filings do you think will be unsealed?

I think a large portion of it is going to be unsealed when the trial starts and as the trial proceeds and certainly things that come into the public arena as a result of in-court presentations — those are all going to be unsealed in advance of that happening because they couldn't be discussed or they couldn't be introduced until that sealing is taken away.

But things that relate to the much bigger picture — the national security picture, the way intelligence is gathered, certain intelligence information regarding Chechnya and potential suspected terrorist groups there — those types of things to the extent that they were the subject of any pleadings are not likely to come out at all.

It's really just going to be on an issue-by-issue, topic-by-topic basis and really only that which is coming out at trial and is being used at trial or the judge is making specific rulings related to, those are what’s going to be unsealed eventually in the course of the trial.

What are some specific examples of things that the public can expect to be unsealed at the trial?

Anything that is going to relate to exhibits that will be presented at trial, lists related to those, some of the things that the government has wanted to hold back for the time being from public awareness, I think that’s going to be released. Judge O'Toole said that the witness lists will be unsealed and will be made public as the trial starts, but out of deference from concerns that some of the witnesses may have about their personal information being shared and the possibility of anything from press to public inquiries about them and also concerns about their own safety he’s holding them back, but that’s the type of thing that’s eventually going to become unsealed as the trial gets underway.

Just to be clear, you believe that in these types of terrorism cases the large volume of sealed documents is not unusual?

No. I don’t think it's unusual at all, particularly if a lot of the requests in the course of discovery went to investigative techniques, foreign intelligence and things like that because these cases do have a national security element, a national security interest. A lot of the agencies involved with our intelligence are going to do everything that they can to maintain the secrecy that relates to things that they are doing that isn't necessarily going to the heart of the Tsarnaev defense or going to be a part of the government’s evidence.

How do the sealed documents relate to the SAMs law, the special administrative measures in place in the Tsarnaev case? [SAMs are rules and protocols concerning how a detainee can communicate, how they are isolated and detained as well as guidelines on visits.]

There is an ongoing concern any time you have a high-profile detainee of his status that there are people out there that have an interest in potentially trying to free him, that there may be communications going back and forth between him and other folks with whom he may be connected who could still be engaged in anti-American or terror activities. Now, there’s no indication that that is so, at least as far as the public knows, but [the special administrative measures] are precautionary measures that are being taken by the U.S. Marshal service to make sure any risk of that is either eliminated or negated.

So how do the Special Administrative Measures affect the amount of sealed documents in this case?

There are certain measures that the marshals don’t want to share with the public at large in order to maintain the high level of security that’s necessary for a high-profile defendant and for that reason there are parts of those special administrative measures that they’re not sharing with the public and that are under seal. And because conditions of detention can be the subject of judicial order or arguments over those conditions under which someone is being detained can be the subject of a judicial order, necessarily how the detention is being performed can be a sensitive issue and is an issue that sometimes for certain elements is kept under seal and kept away from the public eyes because by its very nature the custodians do not want people on the outside to know how they are holding somebody because they don’t want their security to be breached.

What does the sealing of so many documents mean for the trial?

I would say, and I haven’t counted them so I have to be careful, but if you are looking at a third of the filings being placed under seal that is an unusually high amount. Specific showings have to be made and judges in the district of Massachusetts and all over the country are very strict about whether appropriate showing and appropriate need is there and it’s not unusual for a judge to deny or reject motions to seal because there really isn't a basis or a reason to do it. So, if Judge O'Toole is permitting these filings under seal, you can be assured that a proper showing of need has been made by the moving party.