Yes, it is believed that Neanderthals and modern humans were capable of interbreeding with each other. Genetic evidence from the genomes of present-day humans outside of Africa indicates that interbreeding occurred between early modern humans and Neanderthals when they coexisted in regions like Europe and Asia.
This interbreeding likely took place during periods when both Neanderthals and early modern humans lived in close proximity, such as around 50,000 to 60,000 years ago. The genetic evidence suggests that some individuals from these two groups mated and produced offspring with mixed ancestry.
However, it's essential to note that the interbreeding between Neanderthals and modern humans was likely limited, as the interactions between the two species would have depended on various factors such as population sizes, social dynamics, and geographical barriers.
Due to these interbreeding events, traces of Neanderthal DNA can be found in the genomes of non-African modern humans today. When modern humans left Africa and encountered Neanderthals in other parts of the world, they would have had opportunities for interbreeding, leading to the presence of Neanderthal genetic elements in some present-day humans' genomes.
Overall, while Neanderthals and modern humans were different species, they were still close enough genetically to interbreed under certain circumstances.