I believe that in the older books of the bible (Genesis and Job), it at times refers to the "sons of God" as angels.
Genesis 6:4-5
"4 There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
5 And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually."
Job 1:6-7
6 Now there was a day when the sons of God came to present themselves before the Lord, and Satan came also among them.
7 And the Lord said unto Satan, Whence comest thou? Then Satan answered the Lord, and said, From going to and fro in the earth, and from walking up and down in it.
Job 2:1-3
Again there was a day when the sons of God came to present themselves before the Lord, and Satan came also among them to present himself before the Lord.
2 And the Lord said unto Satan, From whence comest thou? And Satan answered the Lord, and said, From going to and fro in the earth, and from walking up and down in it.
3 And the Lord said unto Satan, Hast thou considered my servant Job, that there is none like him in the earth, a perfect and an upright man, one that feareth God, and escheweth evil? and still he holdeth fast his integrity, although thou movedst me against him, to destroy him without cause.
It should be noted that before Christ, Satan and the fallen angels could go and accuse those upon earth before the throne of God as mentioned in the book of Job. When Jesus came, died on the cross, and rose from the dead, he concurred sin and death and the fallen angels could no longer come before the throne of God and accuse the brethren.Revelation 12:9-11 "And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him. And I heard a loud voice saying in heaven, Now is come salvation, and strength, and the kingdom of our God, and the power of his Christ: for the accuser of our brethren is cast down, which accused them before our God day and night. And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death.'
Job 38:5-7
5 Who hath laid the measures thereof, if thou knowest? or who hath stretched the line upon it?
6 Whereupon are the foundations thereof fastened? or who laid the corner stone thereof;
7 When the morning stars sang together, and all the sons of God shouted for joy?
In the epistle of Peter he mentions angels that sinned who are in chains reserved unto judgment, while also mentioning Noah and the flood! So referring to the Genesis passage.
2 Peter 2:3-5
3 And through covetousness shall they with feigned words make merchandise of you: whose judgment now of a long time lingereth not, and their damnation slumbereth not.
4 For if God spared not the angels that sinned, but cast them down to hell, and delivered them into chains of darkness, to be reserved unto judgment;
5 And spared not the old world, but saved Noah the eighth person, a preacher of righteousness, bringing in the flood upon the world of the ungodly;
It also mentions this in Jude.
Jude 1:6
And the angels which kept not their first estate, but left their own habitation, he hath reserved in everlasting chains under darkness unto the judgment of the great day.
Jude also quotes a passage from the book of Enoch 1:9 in verses 14-15
"And Enoch also, the seventh from Adam, prophesied of these, saying, Behold, the Lord cometh with ten thousands of his saints,To execute judgment upon all, and to convince all that are ungodly among them of all their ungodly deeds which they have ungodly committed, and of all their hard speeches which ungodly sinners have spoken against him."
From what I have read in Jude and Peter's epistles, they both seem to have a knowledge of the book of Enoch. In the book of Enoch in chapter 15, it talks about how the angels (also known as watchers) defiled themselves with the daughters of men and it even explains about the giants.
Many people believe that the book of Enoch was corrupted, and that might be so. I just find it interesting how both Peter and Jude have similarities to it. In a history article I read once that the apostles had the book of Enoch with them at the time, so during their lives it must not have been a corrupted version.
Anyway, this is just my thoughts on it. I have studied the scriptures many times on it and I really think that they were fallen angels.