Sixteen people are now known killed in the race riot.

=Sixteen people are now known to have been killed in the race riot.

=It is now known that sixteen people were killed in the race riot.

原句中的killed in the race riot为不定式完成被动式的省略形式，在句中作主语补足语。

本句总体上是know 的被动句，killed in the race riot  为“主语补足语”，改为主动语态，则是“宾语补足语”：

We now know sixteen people  killed in the race riot.

know 在本句表示“查明，搞清楚”之意，可以接复合宾语。

我重新翻译一下：

Sixteen people are now known killed in the race riot.

【翻译】目前已知有16人在种族骚乱中丧生。

Sixteen people are now known killed in the race riot.

=Sixteen people (who were省略) killed in the race riot are known now（定语从句省略关系代词who+助动词were）

=It is known that sixteen were killed in the race riot(主语从句）

=It is known that sixteen have been killed in the race riot.(主语从句）

=People know that sixteen people were killed/have been killed in the race riot（宾语从句）

=Sixteen people are now known (to be 省略) killed in the race riot.（主语补足语）

=Sixteen people are now known to have been killed in the race riot.(主语补足语）

=We know sixteen people to be killed in the race riot（宾语补足语）

=We know sixteen people to have been killed in the race riot（宾语补足语）

关于know搭配如下

(1)know sb to do sth

We've never known him to do anything unjust.
我们从来没有说他做过任何不公正的事情。

I never knew him to act without thinking.
我从未发现他轻率行事过。

(2)sb be known to have done sth

He was known to have taken part in the Long March.
人们都知道他参加过长征。