<s>
Killing	B-Application
Floor	I-Application
is	O
a	O
cooperative	O
first-person	O
shooter	O
video	O
game	O
developed	O
and	O
published	O
by	O
Tripwire	O
Interactive	O
.	O
</s>
<s>
It	O
was	O
originally	O
released	O
as	O
an	O
Unreal	B-Application
Tournament	I-Application
2004	I-Application
mod	O
in	O
2005	O
.	O
</s>
<s>
A	O
full	O
retail	O
release	O
followed	O
on	O
May	O
14	O
,	O
2009	O
,	O
for	O
Microsoft	B-Application
Windows	I-Application
,	O
and	O
for	O
OS	B-Operating_System
X	I-Operating_System
on	O
May	O
5	O
,	O
2010	O
.	O
</s>
<s>
The	O
Linux	B-Application
version	O
was	O
released	O
on	O
the	O
Steam	B-Application
platform	O
in	O
November	O
2012	O
.	O
</s>
<s>
A	O
sequel	O
,	O
Killing	B-Application
Floor	I-Application
2	I-Application
,	O
was	O
announced	O
on	O
May	O
8	O
,	O
2014	O
,	O
and	O
released	O
on	O
November	O
18	O
,	O
2016	O
,	O
worldwide	O
.	O
</s>
<s>
Killing	B-Application
Floor	I-Application
is	O
a	O
first-person	O
shooter	O
with	O
two	O
game	O
modes	O
:	O
Killing	B-Application
Floor	I-Application
and	O
Objective	O
.	O
</s>
<s>
In	O
Killing	B-Application
Floor	I-Application
mode	O
,	O
the	O
player	O
fights	O
waves	O
of	O
zombie-like	O
specimens	O
-	O
or	O
ZEDs	O
-	O
with	O
each	O
wave	O
becoming	O
successively	O
more	O
difficult	O
,	O
until	O
it	O
concludes	O
with	O
a	O
battle	O
against	O
a	O
"	O
boss	O
"	O
specimen	O
called	O
the	O
Patriarch	O
.	O
</s>
<s>
The	O
trader	O
buys	O
and	O
sells	O
ammo	O
,	O
armor	O
,	O
and	O
weapons	O
,	O
such	O
as	O
katanas	B-Operating_System
,	O
pipe	O
bombs	O
,	O
flamethrowers	O
and	O
shotguns	O
.	O
</s>
<s>
A	O
software	B-Application
development	I-Application
kit	I-Application
(	O
SDK	O
)	O
and	O
level	O
editor	O
are	O
included	O
to	O
aid	O
in	O
the	O
creation	O
of	O
modifications	O
and	O
levels	O
.	O
</s>
<s>
Horzine	B-Application
Biotech	O
,	O
a	O
biotechnology	O
company	O
,	O
is	O
contracted	O
to	O
conduct	O
experiments	O
of	O
a	O
military	O
nature	O
involving	O
mass	O
cloning	B-Application
and	O
genetic	O
manipulation	O
.	O
</s>
<s>
Despite	O
the	O
best	O
efforts	O
of	O
the	O
local	O
police	O
,	O
the	O
civilians	O
are	O
quickly	O
overwhelmed	O
and	O
consumed	O
by	O
the	O
seemingly	O
endless	O
supply	O
of	O
clones	O
now	O
streaming	O
from	O
the	O
gaping	O
maw	O
of	O
Horzine	B-Application
Biotech	O
's	O
headquarters	O
.	O
</s>
<s>
Killing	B-Application
Floor	I-Application
was	O
originally	O
a	O
total	O
conversion	O
mod	O
for	O
the	O
game	O
Unreal	B-Application
Tournament	I-Application
2004	I-Application
,	O
first	O
released	O
in	O
2005	O
.	O
</s>
<s>
The	O
lead	O
developer	O
Alex	O
Quick	O
was	O
first	O
approached	O
by	O
Tripwire	O
Interactive	O
about	O
porting	O
Killing	B-Application
Floor	I-Application
to	O
their	O
game	O
Red	B-Application
Orchestra	I-Application
:	I-Application
Ostfront	I-Application
41-45	I-Application
,	O
but	O
Quick	O
did	O
not	O
follow	O
through	O
.	O
</s>
<s>
A	O
few	O
years	O
later	O
,	O
when	O
their	O
user	O
base	O
started	O
to	O
dwindle	O
,	O
Quick	O
contacted	O
Tripwire	O
again	O
to	O
negotiate	O
the	O
port	O
to	O
Red	B-Application
Orchestra	I-Application
,	O
and	O
its	O
distribution	O
via	O
Steam	B-Application
—	O
similar	O
to	O
what	O
Tripwire	O
had	O
already	O
done	O
with	O
another	O
Red	B-Application
Orchestra	I-Application
mod	O
Mare	B-Application
Nostrum	I-Application
—	O
and	O
then	O
later	O
,	O
the	O
retail	O
game	O
.	O
</s>
<s>
The	O
full	O
Killing	B-Application
Floor	I-Application
retail	O
version	O
was	O
announced	O
in	O
March	O
2009	O
.	O
</s>
<s>
In	O
2012	O
,	O
with	O
the	O
launch	O
of	O
the	O
Linux	B-Application
Steam	B-Application
version	O
,	O
the	O
original	O
non-Steam	O
servers	O
were	O
closed	O
and	O
the	O
game	O
was	O
made	O
a	O
Steam	B-Application
exclusive	O
.	O
</s>
<s>
A	O
late	O
2012	O
patch	O
introduced	O
support	O
for	O
the	O
Steam	B-Application
Workshop	O
;	O
a	O
distribution	O
platform	O
for	O
user-made	O
content	O
functioning	O
via	O
the	O
Steam	B-Application
client	I-Application
.	O
</s>
<s>
Killing	B-Application
Floor	I-Application
was	O
the	O
top-selling	O
game	O
on	O
Steam	B-Application
shortly	O
after	O
it	O
was	O
released	O
(	O
as	O
of	O
May	O
19	O
,	O
2009	O
)	O
.	O
</s>
<s>
During	O
the	O
PlayStation	O
Experience	O
event	O
in	O
December	O
2014	O
,	O
Tripwire	O
Interactive	O
announced	O
that	O
Killing	B-Application
Floor	I-Application
had	O
sold	O
almost	O
3	O
million	O
copies	O
on	O
PC	O
.	O
</s>
<s>
Killing	B-Application
Floor	I-Application
has	O
won	O
a	O
number	O
of	O
awards	O
including	O
Overall	O
Best	O
PC	O
Game	O
of	O
2009	O
in	O
Voodoo	O
Extreme	O
's	O
Reader	O
Choice	O
Awards	O
.	O
</s>
<s>
Killing	B-Application
Floor	I-Application
has	O
a	O
Metacritic	O
score	O
of	O
72/100	O
.	O
</s>
<s>
The	O
sequel	O
,	O
Killing	B-Application
Floor	I-Application
2	I-Application
,	O
was	O
released	O
in	O
November	O
2016	O
for	O
Windows	O
and	O
PlayStation	O
4	O
and	O
August	O
2017	O
for	O
Xbox	O
One	O
.	O
</s>
<s>
By	O
May	O
2019	O
,	O
the	O
Killing	B-Application
Floor	I-Application
series	O
has	O
sold	O
more	O
than	O
10	O
million	O
units	O
combined	O
,	O
with	O
total	O
revenues	O
over	O
.	O
</s>
