* All main transit lines from Baumgarten back online
* Russian gas flows to Italy have resumed
* Cause of explosion investigated by two commissions (Adds details Russian gas flows to Italy, Ukraine, gas prices, cause of explosion)
VIENNA, Dec 13 (Reuters) - All main arteries supplying neighboring countries from Austria's main gas pipeline hub were back online before midnight after an explosion there shut it down on Tuesday, the operating company said on Wednesday.
The Baumgarten site in eastern Austria is a major regional transfer node, taking gas from as far away as Russia and pumping it towards neighbors including Italy - its biggest recipient - as well as Germany, Hungary, Slovenia and Croatia.
"We have been back to the same supply levels as before the accident at the Baumgarten compressor station, which means all transit systems in all directions are well functioning and back in operation," Gas Connect Austria said on its website.
Tuesday's explosion, in which one person was killed and 21 were injured, led Italy to declare a state of emergency as flows from the site were cut off for most of the day.
Italian gas grid operator Snam said flows from Russia resumed on Tuesday night.
News of the blast sent gas prices in Europe soaring on fears it would restrict supply as winter sets in, as one third of Russian natural gas is transported via the transport hub, according to Gas Connect.
Italian day-ahead gas prices surged 110 percent to $17.25 per mmBtu on Tuesday.
Russian gas transit through Ukraine dropped 23 percent to 200 million cubic meters per day in the wake of the explosion, Maksim Belyavsky, aide to Ukraine's energy minister, told Reuters on Wednesday, adding that Ukraine was now ready to ramp up pumping to levels seen before the blast.
The cause of the explosion could have been a technical fault, Harald Stindl, co-head of Gas Connect Austria, said at ORF radio.
Work was carried out on the filter system on Monday, including refilling separators, one of which could have had a technical defect that led to the blast, he said.
Two independent probes, one internal by Gas Connect and one by Lower Austria's criminal police, were being conducted to get a clear picture. (Reporting by Kirsti Knolle, Stephen Jewkes, Pavel Polityuk; Editing by Ludwig Burger and John Stonestreet)