Here are some passages I think relevant to the point. Some seem to support the "in-group morality" theory, better than others. I am not yet sure how strong the overall argument for it actually is, though.

It is clear that a distinction between Israelites and foreigners runs all though the Torah. Various verses also make it quite obvious that "neighbors" refers to fellow Israelites. One example is:

Lev 19.17: You shall not hate your fellow countryman in your heart; you may surely reprove your neighbor, but shall not incur sin because of him. You shall not take vengeance, nor bear any grudge against the sons of your people, but you shall love your neighbor as yourself; I am the LORD.

Several of the commandments have plain wordings which make it clear they apply just to actions towards one's neighbors. Why else are such qualifying phrases included at all, and repeated so often? It makes little sense, on the theory that the commandments are referring the the treatment of any human being. The phrasing makes more sense if the Israelites were considered a special class, to be treated better than others.

Ex 20.13-17: Thou shalt not kill, neither shalt thou commit adultery, neither shalt thou steal, neither shalt thou bear false witness against thy neighbour. Neither shall you covet your neighbor's wife, and you shall not desire your neighbor's house, his field, or his manservant, or his maidservant, his ox, or his ass, or anything that is your neighbor's.

The Torah has more explicit cases of different rules being applied to Israelites than to foreigners. For example, it was forbidden to charge a fellow Israelite interest on loans, yet interest could be charged to foreigners.

Duet 23:20 Unto a stranger thou mayest lend upon usury; but unto thy brother thou shalt not lend upon usury: that the LORD thy God may bless thee in all that thou settest thine hand to in the land whither thou goest to possess it.

Note that if the following passage is understood to refer only to the treatment of "one's neighbors", there is no contradiction with the above.

Exodus 22.25-27: If thou lend money to any of my people that is poor by thee, thou shalt not be to him as an usurer, neither shalt thou lay upon him usury. If thou at all take thy neighbour's raiment to pledge, thou shalt deliver it unto him by that the sun goeth down: For that is his covering only, it is his raiment for his skin: wherein shall he sleep? and it shall come to pass, when he crieth unto me, that I will hear; for I am gracious.

Foreigners were not to eat of the passover feast:

Exodus 12:43-45: The LORD said to Moses and Aaron, "This is the ordinance of the Passover: no foreigner is to eat of it; but every man's slave purchased with money, after you have circumcised him, then he may eat of it. A sojourner or a hired servant shall not eat of it.

Animals that die on their own (e.g. of sickness) were not to be eaten by Israelites, but could still be given to foreigners:

Duet 14.21: You shall not eat anything which dies {of itself.} You may give it to the alien who is in your town, so that he may eat it, or you may sell it to a foreigner, for you are a holy people to the LORD your God.

It was fobidden to keep countryman as slaves, but foreigners were okay to possess as slaves:

Leviticus 25.39: If a countryman of yours becomes so poor with regard to you that he sells himself to you, you shall not subject him to a slave's service. He shall be with you as a hired man, as if he were a sojourner; he shall serve with you until the year of jubilee.

Leviticus 25.44-46: As for your male and female slaves whom you may have--you may acquire male and female slaves from the pagan nations that are around you. Then, too, {it is} out of the sons of the sojourners who live as aliens among you that you may gain acquisition, and out of their families who are with you, whom they will have produced in your land; they also may become your possession. You may even bequeath them to your sons after you, to receive as a possession; you can use them as permanent slaves.

Other times the distinction between natives and foreigners is explicitly mentioned, simply to emphasize that it does not make a difference in the case:

Leviticus 24.16: And he that blasphemeth the name of the LORD, he shall surely be put to death, and all the congregation shall certainly stone him: as well the stranger, as he that is born in the land, when he blasphemeth the name of the LORD, shall be put to death.

Various other passages commanded that aliens, living among the Israelites, still be treated well:

Exodus 22.21: You shall not wrong a stranger or oppress him, for you were strangers in the land of Egypt.

Leviticus 19.33-34: When an alien lives with you in your land, do not mistreat him. The alien living with you must be treated as one of your native-born. Love him as yourself, for you were aliens in Egypt.