If you thought Tuesday’s commute across the Casco Bay Bridge was rotten, you’d be right.

But don’t blame the traffic.

That rotten egg smell that wrinkled more than a few drivers’ noses Tuesday morning was caused by construction work going on under the bridge. The smell, similar to natural gas, spread into downtown Portland, where some building owners searched to see if gas lines had ruptured.

A charter school was evacuated until school officials realized the gas smell was stronger outside the building than inside.

The most likely culprit was mercaptan, a chemical that’s added to natural gas to make it smell. Untreated natural gas is odorless, but mercaptan gives it a noxious smell so leaks can be detected.

Unitil, the region’s natural gas utility, said a coal gasification plant, which produced a variant of natural gas from coal, was formerly located on the site west of the bridge.

The plant most likely added mercaptan to the gas to give it an odor and some of the chemical probably leaked into the ground, where it sat until it was disturbed by backhoes installing a new water line Tuesday.

When the construction crew working on the early stages of an expansion of the International Marine Terminal started digging around Tuesday, the mercaptan leaked, giving the impression that a natural gas line had been punctured.

Alec O’Meara, a spokesman for Unitil, said its crews responded to several reports of broken gas lines, only to find that the system was fine.

Police in Portland and South Portland received dozens of calls, but also said reports of broken gas lines were unfounded.

At the new Hyatt Place hotel, the utility and Portland fire crews declared an all-clear before guests had to be evacuated, but the teachers and staff at Baxter Academy on York Street ushered students into nearby parks at about 8:30 a.m.

They had buses on standby ready to take students home, before a Unitil crew told officials about 15 minutes later that there wasn’t a gas leak.

O’Meara said the utility had crews on alert most of Tuesday, particularly around West Commercial Street, where the work was being done to create a storage area for shipping trailers and to connect the terminal with rail lines. The work will continue until late next summer.

That means the smell might recur, said Adam St. Michel, the project manager for Shaw Brothers, which is the lead contractor for the terminal expansion.

St. Michel said the company was aware there might be chemicals in the work area, but said the work crew was taking precautions and wasn’t in any danger.

Mercaptan is harmless and dissipates quickly in the air.

Jean D. MacRae, an associate professor of civil engineering at the University of Maine, said the chemical is often confused with hydrogen sulfide, which smells the same but is a much stronger and more dangerous chemical.

MacRae said it’s also possible the smell came from natural substances in the soil breaking down slowly until they were exposed to oxygen, but she doubts enough organic material has built up at the site for that to be likely.

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