How did the trusts/monopolies Teddy Roosevelt broke up become trusts/monopolies in the first place?

by Soronan_Dogyote

I'm curious what role the government played in their development. If it had an active role, did it knowingly help the companies realize monopolies or was that outcome unintentional? Did the government play a passive role, and simply do nothing while the trusts developed? Was it passive because it didn't realize what was happening, was it convinced to look the other way, or was it neutral?

Thanks!

Soronan_Dogyote

Additional question via /u/Dixnorkel, would any of the characteristics be recognizable today?

Specifically, I'm thinking of Cable/DSL in America, where companies will restrict operation to areas where competitors are not offering service. Effectively, regional monopolies.