What service provider is iPhone when bought in Apple store in US?

What service provider is iPhone when bought in Apple store in US?

When someone buys an iPhone from Apple in the US, is it locked to a certain mobile service provider such as AT&T or is it unlocked for any service provider?

I have a relative going to US from the UK and so there would be no point in buying an iPhone there if they are locked.

Thanks

Added (1). I have just had a look on the Apple US website (i'm from UK), and when you go to select for instance an iPhone 5, apple then directs you to select a Carrier such as AT&T etc. How would I get an unlocked iPhone 5? And where would I get it (apple store, or apple online).

Added (2). There are two comments below this -

The first saying that there are unlocked iPhone 5's on the Apple store US,
The second saying that the only unlocked iPhone 5's is on the Apple store UK.

You can buy a unlocked iPhone via apple
Apple Store and asked for it unlocked
prices: are $649, $749 and $849

I'm sure they are still backed up on pre orders for the next 2-4 weeks from what i heard

IPhones sold through Apple Store can be for the carriers (Sprint, Verizon, AT&T) (they would ask which in that case) or unlocked (not available as I type this).

The factory unlocked iPhones are available in the UK currently. Check the UK Apple site for more about it. You could get them either in store or online.

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