Most international observers believe Syrian President Bashar al-Assad was behind the attack, with the support of his Russian allies.
Moscow's top diplomat accused the West of targeting the Russian public, not elites, with sanctions.
A suspected chemical attack in Douma, Syria, killed dozens, including children, in early April.
The Pentagon has denied responsibility for the attack.
The lawyers, who documented abuses in Syria, are being held by opposing sides in the country's ongoing conflict.