NEWTON — After three months of digging through records dating back to 1887, the city has found 641 residential properties across all 13 villages where service lines bringing water from the municipal main directly into homes could be made of lead, Newton officials announced Wednesday.

The locations were found after staff went through available residential building records, and pulling out any where there was a question that lead pipes may have been used, according to Public Works Commissioner Jim McGonagle. He said the pipes in question were all put in by developers building prior to 1921.

Certified letters are being sent on Thursday to all of the property owners affected, alerting them to the issue. Owners will be asked permission for an engineering firm hired and paid for by the city to come onto their property to determine whether the pipe is actually made of lead, and if so, the scope and cost of replacement.

City officials said that receiving a letter does not mean there are unsafe lead levels in the home’s drinking water. The water, which is provided to Newton by the Massachusetts Water Resources Authority, is tested each year, and treated with chemicals to make lead and copper less likely to leach into water as it flows through pipes.

The cost of replacing the lead service lines, which would have been installed by developers at the time of construction, is the responsibility of the property owners.

However, Mayor Setti Warren said the city has applied for a $4 million interest-free loan from a $100 million fund established by the MWRA for its 34 member cities and towns to replace lead service lines.

“We want to make sure the cost of remediation is not a barrier for anyone to get this work done,’’ Warren said, adding that a “cost-sharing’’ program is being developed, and will be finalized with the City Council.

The MWRA estimates the cost of replacing a residential service line as typically between $3,000 and $5,000, according to city officials.

Warren said this latest finding is part of an unprecedented lead testing program begun three months ago in the aftermath of news from Flint, Mich., where that city’s water supply was found to contain levels of lead above the threshold considered safe.

Warren said he is going far beyond state Department of Environmental Protection regulations, which require the city to test 15 homes, and one water fountain and one faucet in two of the city’s schools each year. In cases where “actionable’’ lead levels are found, the state DEP requires a mitigation plan, but does not require additional testing, McGonagle said.

“We made the decision to do the testing, to do all the additional testing, all the additional investigating, and we’re going to be transparent with what we find,’’ he said. “We’re taking this very seriously, and we should.’’

In addition to the lead service mains, “actionable’’ lead levels were found in five of seven water fountains tested at the Burr Elementary School. Work has begun at Burr to replace the main water service line from the street and re-plumb the entire building with copper water lines.

The city has also turned off water at drinking fountains at its public parks after tests found elevated lead levels at four fountains. All of the public fountains will remain turned off until a remediation plan is in place.

Also, the most recent round of tests on 264 drinking water sources at the city’s schools found a total of six fountains and six faucets spread across nine schools with “actionable’’ lead levels.

All of those faucets and fountains are being replaced, officials said.

Tests on water from 56 drinking fountains in 26 public buildings have also been conducted, with results expected within two weeks.

Warren said the city is committed to repeating this level of testing over the next two years, with those results determining what testing will be done going forward.

Residents who have questions about the lead service line replacement program can e-mail LSRP@newtonma.gov, or call 617-796-1640. In addition, a list of certified laboratories is available at MWRA.com for those interested in having their home tap water tested.

Ellen Ishkanian can be reached at eishkanian@gmail.com.