At least one generally well informed source -- Jeremy Berg (former NIGMS Director, former EiC of Science) -- suggests that there was a a DDoS attack against NIH:

https://bsky.app/profile/jeremymberg.bsky.social/post/3ljfg6...

However, he doesn't provide further details.

More in this thread by Brian Krebs - https://hachyderm.io/@briankrebs@infosec.exchange/1140895564...

NIH Website Down. Is this temporary?

It's back up