Threading model used for ReadIterator?

[jgans]

I am currently using the ngs::ReadIterator to extract sequence data from SRA files. I notice that when I am reading sequence data from an SRA file, there is an extra thread that appears to be part of the SRA toolkit (i.e. under linux, top reports 200% CPU activity for my application). Where is this extra thread coming from, and it there a way to control the number of threads (e.g. environment variable, API call, etc.)?

Thanks!