Someone should point you to the congressional record. I don't have a link but according to serveral members of congress the number is higher than 6.
http://www.freerepublic.com/focus/f-bloggers/1738432/posts
I came up with THIS source that seems to track the allegation at the top of this thread:
http://www.humanevents.com/article.php?id=10663
If you read it, it is based entirely on assuming that each illegal alien here has a propensity to murder equal to the averge murder rate in his/her home country. That's it. Nothing more or less. Which is a pretty preposterous way to try to get a figure. I have no doubt that illegals commit murders. A lot of murders. But 100 is a lot, and so is 4,000. If you want to know where the truth lies in between, this "study" won't do it.
I did find THIS chart (Table 12)
http://www.ojp.usdoj.gov/bjs/pub/pdf/p05.pdf
from a DOJ report (linked from Vdare.com, which is definitely not pro-immigrant), showng that about 18% of the people in jail for murder nationwide are Hispanic. That seems to be a hard number. From there, you can make your own assumptions.
Mine would be that the best figures are that maybe 1/3 of hispanics in this country are illegal (10 million out of 30 million). A pro-immigrant study recently showed that native born hispanics commited crimes at a higher rate than immigrants, legal or illegal. The study was criticized by opponent, but not on this point. In fact, the critics mostly argued that it shows the deleterious long-term effects of current immigration policies, as immigrant groups get worse, not better, over time, as was the case with earlier groups. So -- take your pick. But if we split the difference and say illegals murder at same rate as native hispanics, we get 6% of all murders, or about 1,000. That's a lot. But I think it is a better start at a methodology than the original post.