Only a few years ago, no one dared to excavate anywhere that might produce evidence of this, for the tyranny of Hrdlicka lived on after him for a couple generations.
What is troublesome is just what race or ethnic group these people can be assigned to. Back in the sixties, I changed my history textbook from "Siberia" to "Iberia" as source of first Amerindians, and it is gratifying to know I was right!
Seriously, though, while we can say these people came from the East and were of Solutrean (European) culture, their physical type does seem to lean toward the Asiatic or (Amerindians already in South America?)...making one wonder if a handful of culture-bearers arrived from Europe, but picked up slaves in South America who were the bulk of the (few dozens-to-hundreds) arrivals of some 19500 years ago (from whom the whole N Amer aborig pop descended)...and thus racially the orig settlement was S American Indians, but culturally distinct under its seagoing European elite, which may have lasted only a short time before melting in...
What's likely is that many groups came here, but in relatively small numbers. Europeans crossed by skirting the North Atlantic pack ice. Some Asians did the same along the Nothern Pacific pack ice as well. Africans crossed the Atlantic Narrows, which would have been narrower still during the Pleistocene. The European migrants were likely the source of the Clovis tool tradition. Yes, there are notable differences between Solutrean and Clovis, but no similar stone tool styles exist in Asia. Also, these original populations were likely small.
When the Asians entered later, they encountered people already living in the Americas. If all parties were hunter/gatherers, then the meeting was probably peaceful. (It's the agriculturalists that tend to get violent.) The rapid adoption of the Clovis style tools may indicate trade resulting from such friendly relations. (And there is evidence that the Clovis technology spread out of the Northeast rather than out of the Southwest.) They would have simply interbred with the existing populations. If the Asians were more numerous, then of course their descendents would have a more Asiatic appearence.
The multiple migration theory solves a few sticky problems. One of those is the question of how Paleo-indians got to the tip of South America in such short order. It's presumed that the "migration" was really the pre-historic version of urban sprawl. People slowly diffused into the Americas in search of new hunting grounds. Hunter/gatherer villages can only get so big before they start to overhunt an area. When this happens, the village splits and the new group has to find its own territory. This moves the line of advance several miles every few generations. But this doesn't explain how they could have diffused from Beringia to Monte Verde in 2,500 years. (Or, depending upon what dating one uses, how they could have gone backwards in time!) However, diffusion from the Atlantic Narrows to Monte Verde in 6,500 years is far more believable.