Fast poisson solver threading control

I'm trying to control the number of threads used by the fast poisson solver *_Helmholtz_3D.  The code is compiled with -tbb and -mkl, and I'm using tbb::task_scheduler_init() function to control the thread count.  However, this solver seem to be multithreaded regardless of the number of threads I tell tbb to use.  How can I control this?

MKL Version 11.3.2
ICPC Version 2016.2.062

Thanks!