Update from 3.5.19 to 3.6.15 Failed

Tried doing an update to 3.6.x from 3.5.19 from the terminal and got this horrifying message:
“
Performing one update:

- craft 3.5.19 => 3.6.15

Backup the database? (yes|no) [yes]:yes
Backing up the database … done
Performing update with Composer … Killed
“
php version is 7.4.3

The Composer,json file was changed to require Craft 3.6.15, but looks like no other files were actually changed, there is nothing in the logs and, thankfully, the website is working fine.I have done similar updates on other sites without a problem. Any suggestions for how to troubleshoot this? "Killed" is not much of a starting point.

Thanks!