"The unmonitored and largely unregulated activities of Private Maritime Security Copmanies (PMSCs) off the coast of Somalia," said the letter, "may represent a new potential channel for the flow of arms and ammunition into the region."
The Monitoring Group said the firms had already expanded their business to include the leasing of arms, had brought 7,000 new weapons into the area, and needed to be subject to international regulation.
Doherty of Nexus Consulting said he still hopes for more answers and transparency in the maritime security world.
"Life is precious and if there are folks out there using excessive force they need to be known, they need to be accountable," he said. "It would be nice for a government agency to step up and review this and say this is just or isn't. Thus far we haven't seen that happen."
ABC News' Lauren Effron contributed to this report