President Biden said on Monday that Russian President Vladimir Putin should face a war crimes trial over the reported atrocities in Bucha, Ukraine.

What they're saying: Biden also said he would seek more sanctions on Russia, adding that what's happened is Bucha is "outrageous and everyone's seen it."

• "We have to gather the information, we have to continue to provide Ukraine with the weapons they need to continue the fight, and we have gather all the details" to have a war crimes trial, Biden said.

Driving the news: As Ukrainian forces retook the Kyiv region and other areas over the weekend, officials and independent photographers reported bodies of civilians — some with their hands tied behind their backs — strewn in the streets of the city of Bucha.

• Satellite images show a 45-foot-long trench on the grounds of a church in Bucha, where a mass grave was found after Russian troops withdrew.
• Russia's Defense Ministry has rejected the reports.

Between the lines: War crimes have been historically hard to investigate, and often they're even more challenging to prosecute.

• This is especially true for victims and prosecutors seeking to hold leaders or former leaders accountable for documented atrocities.

The big picture: Ukrainian President Volodymyr Zelensky visited Bucha on Monday, telling reporters there, "We want you to show the world what happened here."

• The European Union has said it is working on a new batch of sanctions against Russia.
• U.S. Ambassador to the UN Linda Thomas-Greenfield on Monday said that the U.S. would seek to suspend Russia from the United Nations Human Rights Council in response to Russia's alleged war crimes in Bucha and other parts of Ukraine.

Go deeper: What counts as a war crime and why they're so hard to prosecute

Editor's note: This story has been updated with additional details throughout.