I met the slavers, turned down their quest and killed them all (after they attacked me for climbing on the memorial), before I found Hannibal and the other slaves and accepting their mission. Can I still complete the quest to help the slaves? I retrieved the picture, but my quest log still shows that I have to kill the slavers. I've crawled all over the memorial and can't find any alive. TIA.

Did you go inside the Lincoln Memorial and kill the slavers there too?

The head bloke lives inside, and until he is dead, you will have that in your log.

Face the steps, and go to the left the door isn't too far.

Yep, cleaned it up both inside and outside.

After retrieving the Lincoln memorial picture, I went back to Hannibal and crew, and got the follow-on message to meet them at the memorial, but so far, I've been back there twice, and they haven't arrived.

They should show up at the subway entrance near the memorial (where those dogs are walking about). Just go there and use the wait function until they show up.

However they might have been killed if you were in the same cell with them when they made there way to the memorial.

They were there. Thanks. I was thinking they'd arrive at the memorial. I'd checked the map marker at the station once, but it must have been too early for them to have shown up.

They don't show up at the memorial in case you didn't kill all the slavers.