If people don't verify and submit the federal frequencies to the database, RR can't create a list, which is why you don't see much in the database. Your best bet is to search the federal bands and create a list as you discover them and hopefully you will submit them when found and verified, there are several resources to help identify what you uncover using both the NAC and Radio IDs.
A few other thoughts, US Capitol Police have a talkgroup on the statewide system, Iowa Statewide Interoperable Communications System (ISICS), which makes me think other federal agencies may be on there too, just not identified or fully encrypted so they can't be identified. My state system is very busy with DHS, DEA, FBI and USMS traffic. The other possibility is they are more active on cell phones than two way radios, especially in areas that are sparsely populated. You might look at each county in the database for additional fed freqs, often times these are used in more locations than reported (submitted) so just because it says Des Moines doesn't mean that is the only place it is used.
You might check other nationwide frequencies like the National Interagency Fire Center, ICE, and CBP, just becuase it doesn't specifically say IOWA doesn't mean they aren't used there. Check the nationwide listings in the database. Also, another great resource is the
Fed Files Blog by
@ChrisP .