How does HPL handle lock?

I'm working on a multi-threaded process to load the data files to a bunch of tables. According to the data files' sizes, some files are loaded through "load" sql scripts, others are loaded through high performance loader scripts. The process will load the tables in an order to satisfy the foreign key constraints. Somehow, when two tables all reference to a same key, and they are loaded at the same time, one of the table's load thread will throw error below: 691: Missing key in referenced table for referential constraint (xxx). 107: ISAM error: record is locked. I added "set lock mode to wait 2" to the sql scripts, and the process works fine when all the data files are loaded through sql scripts. Though HPL jobs are running one by one, a HPL job could be run at the same time when a sql script is running. I could not figure out a way to add "lock mode" statement to HPL load job description. My question is how HPL handles the load job if the job encounters a locked reference key? Will it wait until the locked record is free? Is there any way to set lock mode in a HPL load job? Thanks in advance for your response! Summer