The work was grueling, especially for a former high school French teacher unaccustomed to that type of physical labor.

"It was a huge space," Siby says. "When we had a party of sometimes 2 or 3,000 people, you could spend two days just peeling onions. About 10, 15 pounds of carrots, potatoes to peel."

Sometimes, when he had time off, Siby would walk up to one of the restaurant's legendary, massive windows overlooking the XX. "I remember leaning my back on that glass window, it felt safe and secure."

Down below, on the streets, Siby never felt safe. He was terrified of being denied asylum and sent back to Cote D'Ivoire. He says, up there in the restaurant, on top of the world, he perfected the art of becoming invisible: Don't ask for help, don't complain, don't draw attention.

"This is the reality of an immigrant. You have to be invisible in order to exist. Being out there, it's just exposing yourself. Especially when you are not solid," he says. "You can be picked up any day. It is important for you to be invisible. Never go to the police. You don't want to deal with any government entity."

All the workers in the towers understood this. Being immigrants brought camaraderie behind those kitchen doors. <> The Muslim workers bonded during Ramadan. There were playful wars over what music to play, which the Dominicans always seemed to win, he says.

And there were soccer games on weekends in Queens. It was after one of those games that Siby's friend, Moises Rivas, from Ecuador, asked to swap days. Siby agreed to work the Sunday shift, on Sept. 9th. In exchange, Rivas would take Tuesday, Sept. 11.

He remembers that morning. He got a desperate call from the wife of his roommate- the one who'd gotten him a job at Windows Of The World.

"'Your building is burning. Turn the TV on, a helicopter or a plane hit the building! I'm calling my husband, he's not picking up the phone. Can you help me?'" she asked him. <> "I had two phone numbers for the kitchen, and they started ringing. And they kept on ringing until the building collapsed."

Siby spent the next few days running from one hospital to another throughout Manhattan, searching for his co-workers. As he took calls from their desperate families, he realized they were coming to him because so many were undocumented. They they weren't asking officials for help. <,changed the order, for transition>>

"You have the police asking for ID", Siby says, incredulously. <> "I mean, who in his right mind will go to the police with a fake ID?"

<> Further north, in Midtown Manhattan, at Asociacion Tepeyac, this was precisely the nightmare that was beginning to unfold: how to prove that a person living completely outside the system, was at the site of the attacks.

Teresa Garcia says, they even sent folks to look through missing people's homes, through their personal belongings, "to see if they had a passport, a birth certificate, if they sent money with names [on the order]. And it was real difficult, because people weren't using their real identities."

The 9/11 Victim Compensation Fund made it clear that people could apply regardless of immigration status. This was easier said than done. One of the most comprehensive reports written about undocumented immigrants who went missing during the 9/11 attacks, was put together by Alexandra Delano from the New School, and Benjamin Niennas at Monclair State University.

"This idea that somehow, in a moment of tragedy you can make the life of an undocumented migrant more public, and public services and federal relief services more accessible, that's just not possible", says Niennas. "Once you have been socially and politically isolated, it's extremely hard to trust public agencies again. It is also extremely hard to build the knowledge on how to proceed, how to access them."

Eventually, Tepeyac whittled it's list of 700 or so missing people to 67. That's 67 families <> which had at least one head of household disappear in the attacks. But Tepeyac says only about 12 of those families would come forward and be able to prove the existence of their loved one, and get some aid.

Some of the assistance came from churches, non-profits, and private donors.

At Greenfield Hill Congregational in Connecticut, Rev. Alida Ward says she remembers waking up one morning in mid-December, and walking into the church office. "There was an envelope in my mailbox, and I opened it up... and a check for a quarter million dollars fell out. There was a note that said: 'Please use this for all the people who aren't being reached.' And that was all it said."

The church used the money to create grants for families <>.

There are still 1,106 bodies <,remains? or do they call them bodies?>> that have not been identified from 9/11.

The Mexican consulate alone estimates that 16 nationals died that day. Only 5 are acknowledged at the National Sept. 11 Memorial. <>

For many families of the missing <>, there is simply the desire for an acknowledgment.

Professor Alexandra Delano from The New School says, families live in this limbo "when someone has disappeared, and there is no possibility of confirming that this occurred. That was very difficult for families to deal with."

In fact she says the families often asked for a death certificate, "even if they didn't get compensation or any kind of support."

<>20 years later, many of the families simply want to be left alone. NPR spoke to one woman who has maintained that her father, an undocumented Mexican immigrant, was working at Windows On The World at the time. She declined to be interviewed, but described the process of being told there wasn't enough evidence to prove her father's death, as "humiliating."

For all those affected by the tragedy, the toll - physical and mental- is incalculable. <>

Sekou Siby says he struggles with survivor's guilt. "Somebody who goes to war can be accustomed to seeing friends and buddies dying. But I'm just a restaurant worker, a former French teacher who lost more than 70 people in one day. How do I cope with that?"

He's not sure he did. "I didn't know how to get mental health support. And I didn't also understand the need," he says. "As a Muslim, there is this fatalism, that certain things are bound to happen and you have to live through them, and continue to pray." <>

<,This person appears out of nowhere and doesn't really offer anything new...can we cut?>> "Very honestly there was a lot of distrust", says Doctor Benjamin Luft, the Director and Principal Investigator of the Stony Brook WTC Wellness Program. He says there wasn't a lot of communication about available resources for immigrants in need of assistance- including from community leaders.

Teresa Garcia, from Asociacion Tepeyac, says she too has struggled with a profound sense of loss. She visited the National Sept. 11 Memorial as it was being built. She says "it was like... like a big hole. That's how it felt. It's so difficult to accept that some of our people, they never existed. It was so painful."

Garcia says she has lost contact with most of the families of the missing and the survivors. Except for one person: Chino, the man who showed up at her offices on the morning of 9/11, looking like an apparition, caked in white dust.

"He calls me, on 9/11. Every 9/11," she says.

She's expecting his call this Saturday. She'll ask him how life is in the suburbs <., which he moved to after the attacks. He'll tell her he has trouble breathing, and she'll urge him to go to the doctor- what if it's something he inhaled that day? He'll respond the way he always does: whatever it is, he doesn't want to know. And she knows what he'll say next:

"Oh my God. I am so lucky to be alive."

But he doesn't like to talk about how it happened.

Or the invisible people they both tried so hard to find.

They swear, they were real; 20 years ago, they swear they were right there.

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