Unable to boot Pi 3b+

I am unable to get FreeBSD-13.2-RELEASE-arm64-aarch64 to boot on either 
of two brand new Raspberry Pi 3B+ boards. (Just as a diagnostic, I 
checked that both work perfectly well with Raspberry Pi OS.)  I have 
tried several different brands and sizes of SD card (which all boot fine 
with Raspberry Pi OS). I have redone the image downloads multiple times 
and verified the checksums. I burn images to the SD card on another 
machine using dd with bs=1M. If I mount the prepared SD cards on that 
other machine, I can confirm the files are readable.

I have tried both
/ftp/releases/arm64/aarch64/ISO-IMAGES/13.2/FreeBSD-13.2-RELEASE-arm64-aarch64-RPI.img
and
/ftp/releases/arm64/aarch64/ISO-IMAGES/13.2/FreeBSD-13.2-RELEASE-arm64-aarch64-disc1.iso.

Neither boots.
The first image gives green LED blinks: slow four and then fast seven. 
The second never blinks.

Any advice on what I'm missing would be appreciated.