checking for third party reference for Passport application

[kunaluk]

its taken 5 weeks already since i applied for first british passport (subsequent to naturalisation and attending the citizenship ceremony)

for the passport application, first they took around 2 weeks to send my original documents (previous countrys passport and citizenship certificate) back subsequent to some "preliminary examination" as they told me over the phone when i had called them ... then a week later they sent me a letter saying i forgot to attach the citizenship certificate. so i sent it back again.

now another 2 weeks later when i called them to check the status, they said they have written to some third party for some checks and i should wait for another week or so.

any advice what this could be about and why such a long delay?