Jehovah's Witness have a particular affinity for "Jehovah" as the proper name of God. One of the issues they often point out is that "the LORD," which translates the Tetragrammaton YHVH, is a Jewish superstition that has has somehow corrupted and dishonored the one of whom it speaks.
In their understanding of the Godhood (which is assuredly non-Chalcedonian, and non-Trinitarian), YHVH is a very different character from the more mainstream perspective. In their understanding, God is a force, albeit an approachable one, rather than a person. (Note: In this perspective, Jesus as the son of God is not God, but rather a god or godlike thing.)
Between these two things, I suspect the issue of disagreement was less one of "what actually happened," than of one of terminology - or perhaps the nature of God in this instance. The Biblical narrative (which many JW's reject as being corrupt, preferring instead their own NWT) indicates that Jehovah (whom Trinitarians view as God the Father) was in fact the one doing it. Exodus 11 states:
And Moses went on to say: “This is what Jehovah has said, ‘About midnight I am going out into the midst of Egypt, (New World Translation)
Moses said, “Thus says the LORD: ‘About midnight I will go throughout Egypt" (NET Bible)
The question then, is what there understanding of "the LORD" is. From their perspective, its not God - that is Jehovah. From a Chalcedonian perspective, however, it is a distinction without a difference.