  Ionization by relativistically intense short laser pulses is studied in the
framework of strong-field quantum electrodynamics. Distinctive patterns are
found in the energy probability distributions of photoelectrons. Except of the
already observed patterns, which were studied in Phys. Rev. A {\bf 94}, 013402
(2016), we discover an additional interference-free smooth supercontinuum in
the high-energy portion of the spectrum, reaching tens of kiloelectronovolts.
As we show, the latter is sensitive to the driving field intensity and it can
be detected in a narrow polar-angular window. Once these high-energy electrons
are collected, they can form solitary attosecond pulses. This is particularly
important in light of various applications of attosecond electron beams such as
in ultrafast electron diffraction and crystallography, or in time-resolved
electron microscopy of physical, chemical, and biological processes.
