The book of 1 Peter states (emphasis added):
For Christ also died for sins once for all, the just for the unjust, so that He might bring us to God, having been put to death in the flesh, but made alive in the Spirit; in which also He went and made proclamation to the spirits now in prison, who were once disobedient, when the patience of God kept waiting in the days of Noah, during the construction of the ark, in which a few, that is, eight persons, were brought safely through the water. (1 Peter 3:19-20)This passage has been interpreted many different ways throughout church history. One of the difficulties in interpreting this passage is determining what kind of "spirits" are being referred to. One plausible interpretation is that these spirits are the spirits of fallen angles. It seems strange that Jesus would have specifically made proclamation to those human spirits who were disobedient in the time of Noah. Why would the humans who sinned before the flood be singled out? After the flood, men were still wicked. After the flood, men still practiced immorality. If the "spirits" in this passage are indeed the spirits of fallen angels, then this passage provides another short description of demonic activity shortly before the flood.
Peter continues a similar discussion of this topic in 2 Peter (emphasis added):
For if God did not spare angels when they sinned, but cast them into hell and committed them to pits of darkness, reserved for judgment; and did not spare the ancient world, but preserved Noah, a preacher of righteousness, with seven others, when He brought a flood upon the world of the ungodly; and if He condemned the cities of Sodom and Gomorrah to destruction by reducing them to ashes, having made them an example to those who would live ungodly lives thereafter; and if He rescued righteous Lot, oppressed by the sensual conduct of unprincipled men (for by what he saw and heard that righteous man, while living among them, felt his righteous soul tormented day after day by their lawless deeds), then the Lord knows how to rescue the godly from temptation, and to keep the unrighteous under punishment for the day of judgment, and especially those who indulge the flesh in its corrupt desires and despise authority... (2 Peter 2:4-10)This passage, unlike the previous one, directly mentions angels. Once again, these angels are made mention of right around the time of the flood. Most significantly, the phrase in this passage concerning those who indulge the flesh in its corrupt desires could apply to angels. Such sin was certainly taking place among the ungodly men in Noah's time, in Sodom and Gomorrah, and in Lot's family. Were the angels also practicing immorality?
In both of these passages, Peter writes of strange angelic activity which occurred around the time of the flood. This seems to indicate that that something unusual was taking place when Noah lived on the earth, which means that these two passages could certainly support the interpretation that the sons of God in Genesis 6 are fallen angels (especially the passage from 2 Peter). In other words, if the sons of God in Genesis 6 are angels, then these two passages could be compatible with the claim.
Who Are the Sons of God? (Part 5) - Genesis 6:1-4
Summary of Genesis 4
Summary of Genesis 5
State of the World (2948 BC): Noah's Birth - Genesis 5:28
The Genealogy: Adam to Noah - Genesis 5