Accidents aren't listed in the NTSB database until a preliminary report is issued. Apparently the NTSB is still investigating the 407 accidents. There have been at least 2 fatal 407 accidents, neither of which is on the NTSB site, but which have been reported on the FAA preliminary accident site. The FAA reports accidents as they are reported to it, and the information is often sketchy, and sometimes incorrect. The NTSB only issues a report after a probable cause is determined, or when it's a major airline accident, it may release information while the investigation is still ongoing. It's not unusual for accidents not to be listed on the NTSB site for some weeks if a probable cause is not obvious. From the FAA descriptions, the causes of the 407 crashes probably weren't completely self-evident.