The investigation is very detailed and loan officers are being approached multiple times, sometimes over many weeks for additional documents on lending to specific developers, two of the sources said.
A potential rise in mortgage defaults raises risks for banks and developers.
"The risk of new NPLs (non-performing loans) will remain a threat to banks' asset quality," said rating agency Moody's in a June note.
Commercial banks' non-performing loan ratio stood at 1.67% at the end of June, down from 1.73% at the beginning of this year, according to CBIRC data, though many analysts believe the real number is much higher.