* Shares in motorway firm Autostrade down 25 pct at one stage
* Death toll at 38, state of emergency declared
* Operator says bridge was regularly checked
* Government mulls fine, striping Autostrade of toll concession (Adds sanctions options, details from the scene)
GENOA, Italy, Aug 16 (Reuters) - Italy's biggest toll-road operator came under heavy stock-market attack on Thursday and the government said it could impose a heavy fine or revoke its concession over a bridge collapse that killed at least 38 people.
An 80-meter-long section of the bridge, part of a motorway linking the port city of Genoa with southern France, gave way on Tuesday in busy lunchtime traffic, sending dozens of vehicles into free-fall.
Autostrade per l'Italia, part of Milan-listed international toll-road group Atlantia and owned by the Italy's Benetton family, operated the motorway. It says it made regular, thorough safety checks on the 1.2 km-long viaduct, which was completed in 1967 and overhauled two years ago.
At the disaster scene, cranes moved away chunks of rubble as rescue teams searched for survivors. Genoa's chief prosecutor, Francesco Cozzi, said 10-20 people were still missing.
"We can keep going for another 48 hours in the hope of still being able to find someone," said Pietro Cosola, technical director overseeing the removal of rubble.
The government has rounded on Autostrade, saying it could revoke its concession as it had failed to ensure the viaduct's safety, though investigators have yet to identify the exact cause of the collapse.
"The options we're looking at are the full revocation, the revocation of just the A10 section or a penalty, but a 150 million euro ($170 million) penalty is too low," Deputy Transport Minister Edoardo Rixi told Reuters.
Under the terms of the concession Autostrade can be fined up to 150 million euros.
Atlantia shares fell by as much as 25 percent at one point after being indicated down in pre-open trade by as much as 50 percent. Shares were later down around 22 percent.
Shares in other toll-road firms such as SIAS and ASTM also fell, after Deputy Prime Minister Luigi Di Maio said the state would have to take over the country's motorways if concession-holders could not do the job properly.
"It's not possible that someone pays a toll to die," Di Maio said. "Those who were supposed to do the maintenance work did not do it properly. That bridge should have been closed before this tragedy happened."
"The profits that these companies make as monopolies anger many people ... a lot of money should have been invested in security, but instead it went towards dividends."
A government source suggested Italy may seek to fine Autostrade rather than withdraw its concession because the latter could involve a long legal fight and risk the government being ordered to pay compensation to the company.
Shipping industry sources said the bridge collapse, which severed the main artery feeding Genoas Voltri container terminal, had yet to impact on port operations but was likely in coming months to ripple through the wider supply chain
A spokeswoman for Denmarks AP Moller Maersk, which owns the worlds no. 1 container shipping line, said: "Reaching Genoa will remain challenging in the coming period but main connections from/to Vado and main inland markets will not be affected."
Experts and those who lived near the 50-year-old bridge have said the structure began to encounter problems within two decades of its construction. Pieces of concrete were falling off the bridge as long ago as the 1980s, one resident said.
Italian Prime Minister Giuseppe Conte declared a state of emergency for Genoa, one of Italy's busiest ports, whose main land corridor with France has effectively been severed.
Autostrade said it had relied on world-leading experts when making tests and carrying out inspections on the bridge and that these checks had provided reassuring results.
"These outcomes have formed the basis for maintenance work approved by the Transport Ministry in accordance with the law and the terms of the concession agreement," it said.
A source close to the matter said Autostrade per l'Italia would hold an extraordinary board meeting next week following the disaster. ($1 = 0.8797 euros)
(Additional reporting by Valentina Za, Stefano Bernabei and Lisa Di Giuseppe, Jonathan Saul in London, Writing by Mark Bendeich, Editing by William Maclean and Jon Boyle)