The EU on Monday ordered Estonia and Poland to urgently turn over information on tax rulings, the practice exposed in the LuxLeaks affair of handing out sweetheart tax deals to multinationals.
European regulators also ordered 15 countries, including Germany and France, to provide tax agreements with specific companies.
"We are putting together the puzzle of tax ruling practices in the EU," said Margrethe Vestager, the European Union competition commissioner.
The EU is looking to build on existing probes into the tax dealings of Apple in Ireland, Starbucks in the Netherlands and Amazon and Fiat in the Luxembourg.
The dealings in Luxembourg have been particularly embarrasing for Jean-Claude Juncker, the head of the European Commission who was the small duchy's premier when the deals were made.
The commission in December requested the EU's 28 nations provide information on tax deals made between 2010 and 2013 and only Poland and Estonia failed to respond, the commission said.
If the two countries fail to answer in the next 30 days, the commission can take up the matter with the European Court of Justice, the EU's top court.
The LuxLeaks scandal exposed deals that saved some of the world's largest companies, including Apple, IKEA and Pepsi, billions of dollars in taxes.
Many of these complex tax arrangements involve moving profits across several member states, with non-EU Switzerland also playing a central role.
In addition to the biggest economies in the EU, the commission is seeking further information from Italy, Austria, Belgium, the Czech Republic, Denmark, Finland, Hungary, Lithuania, Portugal, Romania, Slovakia, Spain and Sweden.
The commission had already made the same request to Cyprus, Ireland, Malta, the Netherlands and Britain.
Sources said part of that trove of information could help conclude the ongoing cases, but also dig up new allegations involving more companies.